Make sure the deferred query will be invoked only once

See https://groups.google.com/forum/#!topic/loopbackjs/Ez1cyQ0D0s0

lib/mysql.js

@@ -129,7 +129,7 @@ require('util').inherits(MySQL, SqlConnector);
 MySQL.prototype.query = function (sql, callback) {
   var self = this;
   if (!this.dataSource.connected) {
-    return this.dataSource.on('connected', function () {
+    return this.dataSource.once('connected', function () {
       this.query(sql, callback);
     }.bind(this));
   }