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/stdio.go

@@ -26,8 +26,8 @@ import (
 
 // DialStdIO creates a client on stdin/stdout.
 func DialStdIO(ctx context.Context) (*Client, error) {
-	return newClient(ctx, func(_ context.Context) (net.Conn, error) {
-		return stdioConn{}, nil
+	return newClient(ctx, func(_ context.Context) (ServerCodec, error) {
+		return NewJSONCodec(stdioConn{}), nil
 	})
 }
 
@@ -45,20 +45,8 @@ func (io stdioConn) Close() error {
 	return nil
 }
 
-func (io stdioConn) LocalAddr() net.Addr {
-	return &net.UnixAddr{Name: "stdio", Net: "stdio"}
-}
-
-func (io stdioConn) RemoteAddr() net.Addr {
-	return &net.UnixAddr{Name: "stdio", Net: "stdio"}
-}
-
-func (io stdioConn) SetDeadline(t time.Time) error {
-	return &net.OpError{Op: "set", Net: "stdio", Source: nil, Addr: nil, Err: errors.New("deadline not supported")}
-}
-
-func (io stdioConn) SetReadDeadline(t time.Time) error {
-	return &net.OpError{Op: "set", Net: "stdio", Source: nil, Addr: nil, Err: errors.New("deadline not supported")}
+func (io stdioConn) RemoteAddr() string {
+	return "/dev/stdin"
 }
 
 func (io stdioConn) SetWriteDeadline(t time.Time) error {