Incrementally flush RDB on disk while loading it from a master.

This fixes issue #539.

Basically if there is enough free memory the OS may buffer the RDB file
that the slave transfers on disk from the master. The file may
actually be flused on disk at once by the operating system when it gets
closed by Redis, causing the close system call to block for a long time.

This patch is a modified version of one provided by yoav-steinberg of
@garantiadata (the original version was posted in the issue #539
comments), and tries to flush the OS buffers incrementally (every 8 MB
of loaded data).

src/redis.h

@@ -549,7 +549,9 @@ struct redisServer {
     redisClient *master;     /* Client that is master for this slave */
     int repl_syncio_timeout; /* Timeout for synchronous I/O calls */
     int repl_state;          /* Replication status if the instance is a slave */
-    off_t repl_transfer_left;  /* Bytes left reading .rdb  */
+    off_t repl_transfer_size; /* Size of RDB to read from master during sync. */
+    off_t repl_transfer_read; /* Amount of RDB read from master during sync. */
+    off_t repl_transfer_last_fsync_off; /* Offset when we fsync-ed last time. */
     int repl_transfer_s;     /* Slave -> Master SYNC socket */
     int repl_transfer_fd;    /* Slave -> Master SYNC temp file descriptor */
     char *repl_transfer_tmpfile; /* Slave-> master SYNC temp file name */