Merge pull request #974 from ctiller/arghhh

Reintroduce sleep on backup thread

src/core/iomgr/pollset_posix.c

@@ -66,7 +66,7 @@ static void backup_poller(void *p) {
     gpr_timespec next_poll = gpr_time_add(last_poll, delta);
     grpc_pollset_work(&g_backup_pollset, gpr_time_add(gpr_now(), gpr_time_from_seconds(1)));
     gpr_mu_unlock(&g_backup_pollset.mu);
-    /*gpr_sleep_until(next_poll);*/
+    gpr_sleep_until(next_poll);
     gpr_mu_lock(&g_backup_pollset.mu);
     last_poll = next_poll;
   }