Add a method for waiting for unpublished events

[t-persson]

Applicable Issues

fixes: #41

Description of the Change

This was triggered due to a documentation error in the README plus the fact that we are seeing this issue ourselves.
Basically there was no way to check that events have been published to the broker before exiting the publisher.
This change adds a method for waiting for unpublished events.

Alternate Designs

We could open up the _deliveries and _nacked_deliveries so the users can create their own wait methods, but I feel that this should be a feature in the library.
We could also add this as a step when shutting down the publisher, but I decided against it due to it being a bigger change for users.

Benefits

This will allow users to wait for broker ACK before exiting when publishing events

Possible Drawbacks

None that I can see

Sign-off

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