perf: websocket grid connectivity for all internode communication (#18461)

This PR adds a WebSocket grid feature that allows servers to communicate via 
a single two-way connection.

There are two request types:

* Single requests, which are `[]byte => ([]byte, error)`. This is for efficient small
  roundtrips with small payloads.

* Streaming requests which are `[]byte, chan []byte => chan []byte (and error)`,
  which allows for different combinations of full two-way streams with an initial payload.

Only a single stream is created between two machines - and there is, as such, no
server/client relation since both sides can initiate and handle requests. Which server
initiates the request is decided deterministically on the server names.

Requests are made through a mux client and server, which handles message
passing, congestion, cancelation, timeouts, etc.

If a connection is lost, all requests are canceled, and the calling server will try
to reconnect. Registered handlers can operate directly on byte 
slices or use a higher-level generics abstraction.

There is no versioning of handlers/clients, and incompatible changes should
be handled by adding new handlers.

The request path can be changed to a new one for any protocol changes.

First, all servers create a "Manager." The manager must know its address 
as well as all remote addresses. This will manage all connections.
To get a connection to any remote, ask the manager to provide it given
the remote address using.

```
func (m *Manager) Connection(host string) *Connection
```

All serverside handlers must also be registered on the manager. This will
make sure that all incoming requests are served. The number of in-flight 
requests and responses must also be given for streaming requests.

The "Connection" returned manages the mux-clients. Requests issued
to the connection will be sent to the remote.

* `func (c *Connection) Request(ctx context.Context, h HandlerID, req []byte) ([]byte, error)`
   performs a single request and returns the result. Any deadline provided on the request is
   forwarded to the server, and canceling the context will make the function return at once.

* `func (c *Connection) NewStream(ctx context.Context, h HandlerID, payload []byte) (st *Stream, err error)`
   will initiate a remote call and send the initial payload.

```Go
// A Stream is a two-way stream.
// All responses *must* be read by the caller.
// If the call is canceled through the context,
//The appropriate error will be returned.
type Stream struct {
	// Responses from the remote server.
	// Channel will be closed after an error or when the remote closes.
	// All responses *must* be read by the caller until either an error is returned or the channel is closed.
	// Canceling the context will cause the context cancellation error to be returned.
	Responses <-chan Response

	// Requests sent to the server.
	// If the handler is defined with 0 incoming capacity this will be nil.
	// Channel *must* be closed to signal the end of the stream.
	// If the request context is canceled, the stream will no longer process requests.
	Requests chan<- []byte
}

type Response struct {
	Msg []byte
	Err error
}
```

There are generic versions of the server/client handlers that allow the use of type
safe implementations for data types that support msgpack marshal/unmarshal.

internal/grid/debug.go

@@ -0,0 +1,163 @@
+// Copyright (c) 2015-2023 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package grid
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"time"
+
+	"github.com/minio/mux"
+)
+
+//go:generate stringer -type=debugMsg $GOFILE
+
+// debugMsg is a debug message for testing purposes.
+// may only be used for tests.
+type debugMsg int
+
+const (
+	debugPrint = false
+	debugReqs  = false
+)
+
+const (
+	debugShutdown debugMsg = iota
+	debugKillInbound
+	debugKillOutbound
+	debugWaitForExit
+	debugSetConnPingDuration
+	debugSetClientPingDuration
+	debugAddToDeadline
+)
+
+// TestGrid contains a grid of servers for testing purposes.
+type TestGrid struct {
+	Servers     []*httptest.Server
+	Listeners   []net.Listener
+	Managers    []*Manager
+	Mux         []*mux.Router
+	Hosts       []string
+	cleanupOnce sync.Once
+	cancel      context.CancelFunc
+}
+
+// SetupTestGrid creates a new grid for testing purposes.
+// Select the number of hosts to create.
+// Call (TestGrid).Cleanup() when done.
+func SetupTestGrid(n int) (*TestGrid, error) {
+	hosts, listeners, err := getHosts(n)
+	if err != nil {
+		return nil, err
+	}
+	dialer := &net.Dialer{
+		Timeout: 5 * time.Second,
+	}
+	var res TestGrid
+	res.Hosts = hosts
+	ready := make(chan struct{})
+	ctx, cancel := context.WithCancel(context.Background())
+	res.cancel = cancel
+	for i, host := range hosts {
+		manager, err := NewManager(ctx, ManagerOptions{
+			Dialer: dialer.DialContext,
+			Local:  host,
+			Hosts:  hosts,
+			AuthRequest: func(r *http.Request) error {
+				return nil
+			},
+			AddAuth:      func(aud string) string { return aud },
+			BlockConnect: ready,
+		})
+		if err != nil {
+			return nil, err
+		}
+		m := mux.NewRouter()
+		m.Handle(RoutePath, manager.Handler())
+		res.Managers = append(res.Managers, manager)
+		res.Servers = append(res.Servers, startHTTPServer(listeners[i], m))
+		res.Listeners = append(res.Listeners, listeners[i])
+		res.Mux = append(res.Mux, m)
+	}
+	close(ready)
+	for _, m := range res.Managers {
+		for _, remote := range m.Targets() {
+			if err := m.Connection(remote).WaitForConnect(ctx); err != nil {
+				return nil, err
+			}
+		}
+	}
+	return &res, nil
+}
+
+// Cleanup will clean up the test grid.
+func (t *TestGrid) Cleanup() {
+	t.cancel()
+	t.cleanupOnce.Do(func() {
+		for _, manager := range t.Managers {
+			manager.debugMsg(debugShutdown)
+		}
+		for _, server := range t.Servers {
+			server.Close()
+		}
+		for _, listener := range t.Listeners {
+			listener.Close()
+		}
+	})
+}
+
+// WaitAllConnect will wait for all connections to be established.
+func (t *TestGrid) WaitAllConnect(ctx context.Context) {
+	for _, manager := range t.Managers {
+		for _, remote := range manager.Targets() {
+			if manager.HostName() == remote {
+				continue
+			}
+			if err := manager.Connection(remote).WaitForConnect(ctx); err != nil {
+				panic(err)
+			}
+		}
+	}
+}
+
+func getHosts(n int) (hosts []string, listeners []net.Listener, err error) {
+	for i := 0; i < n; i++ {
+		l, err := net.Listen("tcp", "127.0.0.1:0")
+		if err != nil {
+			if l, err = net.Listen("tcp6", "[::1]:0"); err != nil {
+				return nil, nil, fmt.Errorf("httptest: failed to listen on a port: %v", err)
+			}
+		}
+		addr := l.Addr()
+		hosts = append(hosts, "http://"+addr.String())
+		listeners = append(listeners, l)
+	}
+	return
+}
+
+func startHTTPServer(listener net.Listener, handler http.Handler) (server *httptest.Server) {
+	server = httptest.NewUnstartedServer(handler)
+	server.Config.Addr = listener.Addr().String()
+	server.Listener = listener
+	server.Start()
+	return server
+}