Django-pgpubsub

Fixes include:

- [Set default_auto_field on pgpubsub and pgpubsub.tests app config](https://github.com/Opus10/django-pgpubsub/commit/40623efe43ad5e2545b875f3bc686e3d458c8acc)
- [Add default ordering to the Notification model so that they come in c…](https://github.com/Opus10/django-pgpubsub/commit/ecd95eb1405d544453abe77e36f5719094c12d85)
- [Allow using the same channel on multiple listeners](https://github.com/Opus10/django-pgpubsub/commit/9078236d1d8ca5abb33fdb50da6d1d2aff7af2ef)

Note that upgrading to this version will mean we need to apply a migration (adding the ordering to the Notification model).

Points of note for those upgrading from an earlier version of this library:

- Instead of using `print` to log messages to the console, the Python logging module is now used instead: https://github.com/Opus10/django-pgpubsub/pull/51
- A migration is required to pick up changes to both the `Notification` model and to the test models in the `tests` app.
- DISABLE_SERVER_SIDE_CURSORS should be False. See https://github.com/Opus10/django-pgpubsub/issues/38.


**New-Features**

- In built [logging](https://github.com/Opus10/django-pgpubsub/pull/51) using the Python logging module.
- [Ability to process notifications with backward incompatible changes in django models](https://github.com/Opus10/django-pgpubsub/issues/38): see the "Model migrations" section of the READ ME for more.
- [Reduced memory consumption when consuming notifications](https://github.com/Opus10/django-pgpubsub/issues/38).

**Bug Fixes**

- [Bring back Django 2.2. support](https://github.com/Opus10/django-pgpubsub/pull/28)
- [Fix deserialization of trigger channels](https://github.com/Opus10/django-pgpubsub/pull/34) with some follow up fixes.
- [Recovery handles errors consistently with the usual listener process](https://github.com/Opus10/django-pgpubsub/pull/42)


Special thanks to romank0 for the amazing work done towards this release!

Initial release of the pgpubsub library - a distributed task processing framework for Django built on top of the Postgres NOTIFY/LISTEN protocol.