Transaction Status listener list gets overwritten

Transaction status listener is not called when using Gremlin Server

• Caused by the transaction listener list initialization in AbstractThreadLocalTransaction.

• There are 2 different threads in action during the Gremlin Server initialization.

1. The Groovy startup script evaluation which adds the listener
2. The server itself handles the transaction commit or rollback. Both threads end up calling on the ThreadLocal initialValue() which returns new ArrayList() and effectively wipes out the listener.

• Fix is to initialize the list once.

    protected final ThreadLocal<List<Consumer<Transaction.Status>>> transactionListeners = new ThreadLocal<List<Consumer<Transaction.Status>>>() {
        private List<Consumer<Transaction.Status>> list = new ArrayList<>();
        @Override
        protected List<Consumer<Transaction.Status>> initialValue() {
            return list;
        }
    };