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.

cmd/routers.go

@@ -20,13 +20,14 @@ package cmd
 import (
 	"net/http"
 
+	"github.com/minio/minio/internal/grid"
 	"github.com/minio/mux"
 )
 
 // Composed function registering routers for only distributed Erasure setup.
 func registerDistErasureRouters(router *mux.Router, endpointServerPools EndpointServerPools) {
 	// Register storage REST router only if its a distributed setup.
-	registerStorageRESTHandlers(router, endpointServerPools)
+	registerStorageRESTHandlers(router, endpointServerPools, globalGrid.Load())
 
 	// Register peer REST router only if its a distributed setup.
 	registerPeerRESTHandlers(router)
@@ -38,7 +39,10 @@ func registerDistErasureRouters(router *mux.Router, endpointServerPools Endpoint
 	registerBootstrapRESTHandlers(router)
 
 	// Register distributed namespace lock routers.
-	registerLockRESTHandlers(router)
+	registerLockRESTHandlers()
+
+	// Add grid to router
+	router.Handle(grid.RoutePath, adminMiddleware(globalGrid.Load().Handler(), noGZFlag, noObjLayerFlag))
 }
 
 // List of some generic middlewares which are applied for all incoming requests.