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/storage-rest-common.go

@@ -17,6 +17,8 @@
 
 package cmd
 
+//go:generate msgp -file $GOFILE -unexported
+
 const (
 	storageRESTVersion       = "v50" // Added DiskInfo metrics query
 	storageRESTVersionPrefix = SlashSeparator + storageRESTVersion
@@ -25,64 +27,58 @@ const (
 
 const (
 	storageRESTMethodHealth      = "/health"
-	storageRESTMethodDiskInfo    = "/diskinfo"
-	storageRESTMethodNSScanner   = "/nsscanner"
 	storageRESTMethodMakeVol     = "/makevol"
 	storageRESTMethodMakeVolBulk = "/makevolbulk"
-	storageRESTMethodStatVol     = "/statvol"
 	storageRESTMethodDeleteVol   = "/deletevol"
 	storageRESTMethodListVols    = "/listvols"
 
 	storageRESTMethodAppendFile     = "/appendfile"
 	storageRESTMethodCreateFile     = "/createfile"
 	storageRESTMethodWriteAll       = "/writeall"
-	storageRESTMethodWriteMetadata  = "/writemetadata"
-	storageRESTMethodUpdateMetadata = "/updatemetadata"
-	storageRESTMethodDeleteVersion  = "/deleteversion"
 	storageRESTMethodReadVersion    = "/readversion"
 	storageRESTMethodReadXL         = "/readxl"
-	storageRESTMethodRenameData     = "/renamedata"
-	storageRESTMethodCheckParts     = "/checkparts"
 	storageRESTMethodReadAll        = "/readall"
 	storageRESTMethodReadFile       = "/readfile"
 	storageRESTMethodReadFileStream = "/readfilestream"
 	storageRESTMethodListDir        = "/listdir"
-	storageRESTMethodDeleteFile     = "/deletefile"
 	storageRESTMethodDeleteVersions = "/deleteverions"
 	storageRESTMethodRenameFile     = "/renamefile"
 	storageRESTMethodVerifyFile     = "/verifyfile"
-	storageRESTMethodWalkDir        = "/walkdir"
 	storageRESTMethodStatInfoFile   = "/statfile"
 	storageRESTMethodReadMultiple   = "/readmultiple"
 	storageRESTMethodCleanAbandoned = "/cleanabandoned"
 )
 
 const (
-	storageRESTVolume         = "volume"
-	storageRESTVolumes        = "volumes"
-	storageRESTDirPath        = "dir-path"
-	storageRESTFilePath       = "file-path"
-	storageRESTForceDelMarker = "force-delete-marker"
-	storageRESTVersionID      = "version-id"
-	storageRESTReadData       = "read-data"
-	storageRESTTotalVersions  = "total-versions"
-	storageRESTSrcVolume      = "source-volume"
-	storageRESTSrcPath        = "source-path"
-	storageRESTDstVolume      = "destination-volume"
-	storageRESTDstPath        = "destination-path"
-	storageRESTOffset         = "offset"
-	storageRESTLength         = "length"
-	storageRESTCount          = "count"
-	storageRESTPrefixFilter   = "prefix"
-	storageRESTForwardFilter  = "forward"
-	storageRESTRecursive      = "recursive"
-	storageRESTReportNotFound = "report-notfound"
-	storageRESTBitrotAlgo     = "bitrot-algo"
-	storageRESTBitrotHash     = "bitrot-hash"
-	storageRESTDiskID         = "disk-id"
-	storageRESTForceDelete    = "force-delete"
-	storageRESTGlob           = "glob"
-	storageRESTScanMode       = "scan-mode"
-	storageRESTMetrics        = "metrics"
-	storageRESTNoPersistence  = "no-persistence"
+	storageRESTVolume        = "volume"
+	storageRESTVolumes       = "volumes"
+	storageRESTDirPath       = "dir-path"
+	storageRESTFilePath      = "file-path"
+	storageRESTVersionID     = "version-id"
+	storageRESTReadData      = "read-data"
+	storageRESTTotalVersions = "total-versions"
+	storageRESTSrcVolume     = "source-volume"
+	storageRESTSrcPath       = "source-path"
+	storageRESTDstVolume     = "destination-volume"
+	storageRESTDstPath       = "destination-path"
+	storageRESTOffset        = "offset"
+	storageRESTLength        = "length"
+	storageRESTCount         = "count"
+	storageRESTBitrotAlgo    = "bitrot-algo"
+	storageRESTBitrotHash    = "bitrot-hash"
+	storageRESTDiskID        = "disk-id"
+	storageRESTForceDelete   = "force-delete"
+	storageRESTGlob          = "glob"
+	storageRESTMetrics       = "metrics"
 )
+
+type nsScannerOptions struct {
+	DiskID   string          `msg:"id"`
+	ScanMode int             `msg:"m"`
+	Cache    *dataUsageCache `msg:"c"`
+}
+
+type nsScannerResp struct {
+	Update *dataUsageEntry `msg:"u"`
+	Final  *dataUsageCache `msg:"f"`
+}