Race condition between ack/nack (mode client/client-individual) and disconnects (+ Fix provided)

[carantunes]

I identified a race condition when there's some load of processing messages and a disconnect occurs.
This issue only occurs with ack mode client/client-individual which means a ACK or NACK frame will be sent to the server.

In such cases, between the milliseconds a disconnect begins and concludes, some other threads might start processing a new message and when trying to send the frame ACK/NACK will result in either a native BrokenPipeError (unhandled by stompy) or a org.apache.activemq.transport.stomp.ProtocolException: Not connected (properly handled).

I'm providing a MR associated with the issue, showcasing the issue in a new test case and a solution.
For the test case I rely on repetition given that the issue is a race condition.

For the solution I suggest the addition of a wait parameter to the disconnect method, which allows instantly stop receiving and enables wait for current threads to finish already started acks/nacks before stoping executing and closing the socket.