Refactor Federation Destination Queues (#2807)

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

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