fix(syncer): start syncer pool before broker pool

As the latter depends on the former.

apps/emqx/src/emqx_broker_sup.erl

@@ -66,4 +66,4 @@ init([]) ->
         modules => [emqx_broker_helper]
     },
 
-    {ok, {{one_for_all, 0, 1}, [BrokerPool, SyncerPool, SharedSub, Helper]}}.
+    {ok, {{one_for_all, 0, 1}, [SyncerPool, BrokerPool, SharedSub, Helper]}}.