LATENCY DOCTOR first implementation complete.

src/rdb.c

@@ -745,6 +745,7 @@ int rdbSaveBackground(char *filename) {
     } else {
         /* Parent */
         server.stat_fork_time = ustime()-start;
+        server.stat_fork_rate = (double) zmalloc_used_memory() * 1000000 / server.stat_fork_time / (1024*1024*1024); /* GB per second. */
         latencyAddSampleIfNeeded("fork",server.stat_fork_time/1000);
         if (childpid == -1) {
             server.lastbgsave_status = REDIS_ERR;