BREAKING CHANGE: implementation of idempotency

This PR aims to implement Idempotency in the lib consumers.

The most important test scenarios considered are:

I added an example of how to use the consumer in the README.md. (link)

I set PostgreSQL as the default test database for the unique function to work. (link)

To simplify the testing process, I removed the tox implementation that was unnecessarily complex. (link)

New indexes were added to the models. (link)

The Payload class received new functionalities. (link)

Several new tests were created and many were modified.

juntossomosmais / django-outbox-pattern