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_test.go

@@ -20,12 +20,11 @@ package cmd
 import (
 	"bytes"
 	"context"
-	"net/http/httptest"
 	"reflect"
 	"runtime"
 	"testing"
 
-	"github.com/minio/mux"
+	"github.com/minio/minio/internal/grid"
 	xnet "github.com/minio/pkg/v2/net"
 )
 
@@ -437,17 +436,21 @@ func testStorageAPIRenameFile(t *testing.T, storage StorageAPI) {
 	}
 }
 
-func newStorageRESTHTTPServerClient(t *testing.T) *storageRESTClient {
+func newStorageRESTHTTPServerClient(t testing.TB) *storageRESTClient {
+	// Grid with 2 hosts
+	tg, err := grid.SetupTestGrid(2)
+	if err != nil {
+		t.Fatalf("SetupTestGrid: %v", err)
+	}
+	t.Cleanup(tg.Cleanup)
 	prevHost, prevPort := globalMinioHost, globalMinioPort
 	defer func() {
 		globalMinioHost, globalMinioPort = prevHost, prevPort
 	}()
+	// tg[0] = local, tg[1] = remote
 
-	router := mux.NewRouter()
-	httpServer := httptest.NewServer(router)
-	t.Cleanup(httpServer.Close)
-
-	url, err := xnet.ParseHTTPURL(httpServer.URL)
+	// Remote URL
+	url, err := xnet.ParseHTTPURL(tg.Servers[1].URL)
 	if err != nil {
 		t.Fatalf("unexpected error %v", err)
 	}
@@ -464,11 +467,18 @@ func newStorageRESTHTTPServerClient(t *testing.T) *storageRESTClient {
 		t.Fatalf("UpdateIsLocal failed %v", err)
 	}
 
-	registerStorageRESTHandlers(router, []PoolEndpoints{{
+	// Register handlers on newly created servers
+	registerStorageRESTHandlers(tg.Mux[0], []PoolEndpoints{{
 		Endpoints: Endpoints{endpoint},
-	}})
+	}}, tg.Managers[0])
+	registerStorageRESTHandlers(tg.Mux[1], []PoolEndpoints{{
+		Endpoints: Endpoints{endpoint},
+	}}, tg.Managers[1])
 
-	restClient := newStorageRESTClient(endpoint, false)
+	restClient, err := newStorageRESTClient(endpoint, false, tg.Managers[0])
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	return restClient
 }