rpc: implement full bi-directional communication (#18471)

New APIs added:

    client.RegisterName(namespace, service) // makes service available to server
    client.Notify(ctx, method, args...)     // sends a notification
    ClientFromContext(ctx)                  // to get a client in handler method

This is essentially a rewrite of the server-side code. JSON-RPC
processing code is now the same on both server and client side. Many
minor issues were fixed in the process and there is a new test suite for
JSON-RPC spec compliance (and non-compliance in some cases).

List of behavior changes:

- Method handlers are now called with a per-request context instead of a
  per-connection context. The context is canceled right after the method
  returns.
- Subscription error channels are always closed when the connection
  ends. There is no need to also wait on the Notifier's Closed channel
  to detect whether the subscription has ended.
- Client now omits "params" instead of sending "params": null when there
  are no arguments to a call. The previous behavior was not compliant
  with the spec. The server still accepts "params": null.
- Floating point numbers are allowed as "id". The spec doesn't allow
  them, but we handle request "id" as json.RawMessage and guarantee that
  the same number will be sent back.
- Logging is improved significantly. There is now a message at DEBUG
  level for each RPC call served.

rpc/ipc.go

@@ -25,17 +25,17 @@ import (
 )
 
 // ServeListener accepts connections on l, serving JSON-RPC on them.
-func (srv *Server) ServeListener(l net.Listener) error {
+func (s *Server) ServeListener(l net.Listener) error {
 	for {
 		conn, err := l.Accept()
 		if netutil.IsTemporaryError(err) {
-			log.Warn("IPC accept error", "err", err)
+			log.Warn("RPC accept error", "err", err)
 			continue
 		} else if err != nil {
 			return err
 		}
-		log.Trace("IPC accepted connection")
-		go srv.ServeCodec(NewJSONCodec(conn), OptionMethodInvocation|OptionSubscriptions)
+		log.Trace("Accepted RPC connection", "conn", conn.RemoteAddr())
+		go s.ServeCodec(NewJSONCodec(conn), OptionMethodInvocation|OptionSubscriptions)
 	}
 }
 
@@ -46,7 +46,11 @@ func (srv *Server) ServeListener(l net.Listener) error {
 // The context is used for the initial connection establishment. It does not
 // affect subsequent interactions with the client.
 func DialIPC(ctx context.Context, endpoint string) (*Client, error) {
-	return newClient(ctx, func(ctx context.Context) (net.Conn, error) {
-		return newIPCConnection(ctx, endpoint)
+	return newClient(ctx, func(ctx context.Context) (ServerCodec, error) {
+		conn, err := newIPCConnection(ctx, endpoint)
+		if err != nil {
+			return nil, err
+		}
+		return NewJSONCodec(conn), err
 	})
 }