This is a refactor of the federation destination queues.
It fixes a few things, namely:
- actually retry outgoing events with backoff behaviour
- obtain enough events from the database to fill messages as much as
possible
- minimize the amount of running goroutines
- use pure timers for backoff
- don't restart queue unless necessary
- close the background task when backing off
- increase max edus in a transaction to match the spec
- cleanup timers more aggresively to reduce memory usage
- add jitter to backoff timers to reduce resource spikes
- add a bunch of tests (with real and fake databases) to ensure
everything is working
This fixes some edge cases where federation queue backoffs and
blacklisting weren't behaving as expected.
It also adds new tests for the federation queues to ensure their
behaviour continues to work correctly.
