Send ACK to master once every second.

ACKs can be also used as a base for synchronous replication. However in
that case they'll be explicitly requested by the master when the client
sends a request that needs to be replicated synchronously.

src/replication.c

@@ -1441,6 +1441,10 @@ void replicationCron(void) {
             redisLog(REDIS_NOTICE,"MASTER <-> SLAVE sync started");
         }
     }
+
+    /* Send ACK to master from time to time. */
+    if (server.masterhost && server.master)
+        replicationSendAck();
     
     /* If we have attached slaves, PING them from time to time.
      * So slaves can implement an explicit timeout to masters, and will