Throttle BGSAVE attempt on saving error.

When a BGSAVE fails, Redis used to flood itself trying to BGSAVE at
every next cron call, that is either 10 or 100 times per second
depending on configuration and server version.

This commit does not allow a new automatic BGSAVE attempt to be
performed before a few seconds delay (currently 5).

This avoids both the auto-flood problem and filling the disk with
logs at a serious rate.

The five seconds limit, considering a log entry of 200 bytes, will use
less than 4 MB of disk space per day that is reasonable, the sysadmin
should notice before of catastrofic events especially since by default
Redis will stop serving write queries after the first failed BGSAVE.

This fixes issue #849

src/rdb.c

@@ -722,6 +722,7 @@ int rdbSaveBackground(char *filename) {
     if (server.rdb_child_pid != -1) return REDIS_ERR;
 
     server.dirty_before_bgsave = server.dirty;
+    server.lastbgsave_try = time(NULL);
 
     start = ustime();
     if ((childpid = fork()) == 0) {