Do not send the complete payload for the TriggerChannel that stores notification in the DB

[romank0]

Currently for the updated django entity that has an associated TriggerChannel the notification payload is sent via postgres NOTIFY and it contains the old and the new serialized entity state. On the listener the payload sent via postgres channel is used only to find the appropriate Notification record from the DB and then the payload from the record is used.

This sending of the complete payload is suboptimal:

• the serialized size of the record that can be sent is limited by the max payload size (8k) that can be sent via the channel. Given that both old and new records are sent that size is rather small
• the serialized payload makes roundtrips from postgres to application (in the NOTIFY message) and then back to server (as a query parameter)

The better solution seems to be to send only the notification ID for the TriggerChannel that has a persistent notifications.

[romank0]

I have an implementation of this but it depends and is based on PRs that are not yet merged https://github.com/Opus10/django-pgpubsub/pull/66 and https://github.com/Opus10/django-pgpubsub/pull/73 and there might be some amendments to the approach there so I'll add a PR for this when it will be clear what migration path will be chosen there.