Fix a problem where the RabbitMQPublisher could deadlock

[t-persson]

Applicable Issues

Description of the Change

There have been multiple problems with this code. One problem is that it would send events from two different threads. The main thread and the thread that resends nacked events. This could cause the RabbitMQ server to disconnect the client.
Secondly since the send_event method would block and wait for the client to reconnect in case of disconnects, the resend_nacked_events method could block the IOLoop which would prevent the client from reconnecting, thus causing a deadlock

Alternate Designs

I thought about having a queue of events instead which send_event and _resend_nacked_events would populate and having a separate thread go through this queue and send events.
This was a part of the previous design of this library and it had multiple problems due to the complexity that threading adds when using pika.

Benefits

Deadlocks are dangerous and we don't want to have to reconnect because we behave badly.

Possible Drawbacks

This adds more complexity and overhead.

Sign-off

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Signed-off-by: Tobias Persson [email protected]

[t-persson]

Must protect class attributes within send_event as well.

[t-persson]

Copyright notice needs an update of the year