MXS-2179 Activate WD workaround when fetching users

Fetching the users may potentially take longer than the watchdog
timeout. To ensure that MaxScale is not killed by systemd, we must
ensure that the notifications are generated also when MaxScale
synchronously is fetching the users.

server/core/service.cc

@@ -1422,6 +1422,7 @@ void dListListeners(DCB* dcb)
 
 bool Service::refresh_users()
 {
+    mxs::WatchdogWorkaround workaround;
     bool ret = true;
     int self = mxs_rworker_get_current_id();
     mxb_assert(self >= 0);