feat: add isactive check for transaction (#164)

lib/transaction.js

@@ -134,3 +134,10 @@ Transaction.begin = function(connector, options, cb) {
   });
   if (cb.promise) return cb.promise;
 };
+
+/**
+ * Check whether a transaction has an active connection.
+ */
+Transaction.prototype.isActive = function() {
+  return !!this.connection;
+};