Turn off TCP_NODELAY on the slave socket after SYNC.

Further details from @antirez:

It was reported by @StopForumSpam on Twitter that the Redis replication
link was strangely using multiple TCP packets for multiple commands.
This wastes a lot of bandwidth and is due to the TCP_NODELAY option we
enable on the socket after accepting a new connection.

However the master -> slave channel is a one-way channel since Redis
replication is asynchronous, so there is no point in trying to reduce
the latency, we should aim to reduce the bandwidth. For this reason this
commit introduces the ability to disable the nagle algorithm on the
socket after a successful SYNC.

This feature is off by default because the delay can be up to 40
milliseconds with normally configured Linux kernels.

src/redis.h

@@ -633,6 +633,7 @@ struct redisServer {
     int repl_serve_stale_data; /* Serve stale data when link is down? */
     int repl_slave_ro;          /* Slave is read only? */
     time_t repl_down_since; /* Unix time at which link with master went down */
+    int slave_tcp_nodelay_off; /* turn off slave's tcp nodelay */
     int slave_priority;             /* Reported in INFO and used by Sentinel. */
     /* Limits */
     unsigned int maxclients;        /* Max number of simultaneous clients */