patch 7.4.1493

Problem:    Wrong callback invoked for zero-id messages.
Solution:   Don't use the first one-time callback when the sequence number
            doesn't match.

src/testdir/test_channel.py

@@ -143,6 +143,11 @@ class ThreadedTCPRequestHandler(socketserver.BaseRequestHandler):
                         print("sending: {}".format(cmd))
                         self.request.sendall(cmd.encode('utf-8'))
                         response = ""
+                    elif decoded[1] == 'send zero':
+                        cmd = '[0,"zero index"]'
+                        print("sending: {}".format(cmd))
+                        self.request.sendall(cmd.encode('utf-8'))
+                        response = "sent zero"
                     elif decoded[1] == 'close me':
                         print("closing")
                         self.request.close()