Restructure sending messages

Use an internal buffer in pgproto3.Frontend and pgproto3.Backend instead
of directly writing to the underlying net.Conn. This will allow tracing
messages as well as simplify pipeline mode.

internal/pgmock/pgmock.go

@@ -97,7 +97,8 @@ type sendMessageStep struct {
 }
 
 func (e *sendMessageStep) Step(backend *pgproto3.Backend) error {
-	return backend.Send(e.msg)
+	backend.Send(e.msg)
+	return backend.Flush()
 }
 
 func SendMessage(msg pgproto3.BackendMessage) Step {