Diskless SYNC: fix RDB EOF detection.

RDB EOF detection was relying on the final part of the RDB transfer to
be a magic 40 bytes EOF marker. However as the slave is put online
immediately, and because of sockets timeouts, the replication stream is
actually contiguous with the RDB file.

This means that to detect the EOF correctly we should either:

1) Scan all the stream searching for the mark. Sucks CPU-wise.
2) Start to send the replication stream only after an acknowledge.
3) Implement a proper chunked encoding.

For now solution "2" was picked, so the master does not start to send
ASAP the stream of commands in the case of diskless replication. We wait
for the first REPLCONF ACK command from the slave, that certifies us
that the slave correctly loaded the RDB file and is ready to get more
data.

src/redis.h

@@ -493,6 +493,7 @@ typedef struct redisClient {
     int flags;              /* REDIS_SLAVE | REDIS_MONITOR | REDIS_MULTI ... */
     int authenticated;      /* when requirepass is non-NULL */
     int replstate;          /* replication state if this is a slave */
+    int repl_put_online_on_ack; /* Install slave write handler on ACK. */
     int repldbfd;           /* replication DB file descriptor */
     off_t repldboff;        /* replication DB file offset */
     off_t repldbsize;       /* replication DB file size */