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

go.mod

@@ -50,6 +50,7 @@ require (
 	golang.org/x/term v0.0.0-20220919170432-7a66f970e087
 	gopkg.in/h2non/bimg.v1 v1.1.9
 	gopkg.in/yaml.v2 v2.4.0
+	gotest.tools/v3 v3.4.0
 	nhooyr.io/websocket v1.8.7
 )
 
@@ -127,7 +128,6 @@ require (
 	gopkg.in/macaroon.v2 v2.1.0 // indirect
 	gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
-	gotest.tools/v3 v3.4.0 // indirect
 )
 
 go 1.18