fix pipelined WAIT performance issue.

If client gets blocked again in `processUnblockedClients`, redis will not send
`REPLCONF GETACK *` to slaves untill next eventloop, so the client will be
blocked for 100ms by default(10hz) if no other file event fired.

move server.get_ack_from_slaves sinppet after `processUnblockedClients`, so
that both the first WAIT command that puts client in blocked context and the
following WAIT command processed in processUnblockedClients would trigger
redis-sever to send `REPLCONF GETACK *`, so that the eventloop would get
`REPLCONG ACK <reploffset>` from slaves and unblocked ASAP.

src/server.c

@@ -2110,6 +2110,19 @@ void beforeSleep(struct aeEventLoop *eventLoop) {
     if (server.active_expire_enabled && server.masterhost == NULL)
         activeExpireCycle(ACTIVE_EXPIRE_CYCLE_FAST);
 
+    /* Unblock all the clients blocked for synchronous replication
+     * in WAIT. */
+    if (listLength(server.clients_waiting_acks))
+        processClientsWaitingReplicas();
+
+    /* Check if there are clients unblocked by modules that implement
+     * blocking commands. */
+    if (moduleCount()) moduleHandleBlockedClients();
+
+    /* Try to process pending commands for clients that were just unblocked. */
+    if (listLength(server.unblocked_clients))
+        processUnblockedClients();
+
     /* Send all the slaves an ACK request if at least one client blocked
      * during the previous event loop iteration. */
     if (server.get_ack_from_slaves) {
@@ -2125,19 +2138,6 @@ void beforeSleep(struct aeEventLoop *eventLoop) {
         server.get_ack_from_slaves = 0;
     }
 
-    /* Unblock all the clients blocked for synchronous replication
-     * in WAIT. */
-    if (listLength(server.clients_waiting_acks))
-        processClientsWaitingReplicas();
-
-    /* Check if there are clients unblocked by modules that implement
-     * blocking commands. */
-    if (moduleCount()) moduleHandleBlockedClients();
-
-    /* Try to process pending commands for clients that were just unblocked. */
-    if (listLength(server.unblocked_clients))
-        processUnblockedClients();
-
     /* Send the invalidation messages to clients participating to the
      * client side caching protocol in broadcasting (BCAST) mode. */
     trackingBroadcastInvalidationMessages();