database: Fix event emitter warning

the connection object gets reused after release. this means that we keep
attaching the 'error' event and not unlistening.

--trace-warnings can be added to box.service to get the stack trace

src/database.js

@@ -145,7 +145,9 @@ function rollback(connection, transactionError, callback) {
     connection.rollback(function (error) {
         if (error) debug('rollback: error when rolling back', error);
 
+        connection.removeAllListeners('error');
         connection.release();
+
         callback(transactionError);
     });
 }
@@ -169,7 +171,9 @@ function transaction(queries, callback) {
             connection.commit(function (error) {
                 if (error) return rollback(connection, error, callback);
 
+                connection.removeAllListeners('error');
                 connection.release();
+
                 callback(null, results);
             });
         });