mail: use a lock to protect container recreation

needs a lock because the cert code also restart mail server from tasks

src/locks.js

@@ -13,7 +13,9 @@ exports = module.exports = {
     TYPE_APP_PREFIX: 'app_',
     TYPE_UPDATE: 'update',
     TYPE_UPDATE_TASK: 'update_task',
-    TYPE_BACKUP_TASK: 'backup_task'
+    TYPE_BACKUP_TASK: 'backup_task',
+
+    TYPE_MAIL_SERVER_RESTART: 'mail_restart',
 };
 
 const assert = require('assert'),