maintain `connection.state` instead

@mickhansen I believe that problem was occurring because
“protocol_error” is a faulty state, not a valid one.

lib/dialects/mysql/connection-manager.js

@@ -74,7 +74,7 @@ ConnectionManager.prototype.disconnect = function(connection) {
   });
 };
 ConnectionManager.prototype.validate = function(connection) {
-  return connection && ! connection._protocol._ended;
+  return connection && [ 'disconnected', 'protocol_error' ].indexOf( connection.state ) !== -1;
 };
 
 module.exports = ConnectionManager;