bugfix: E2EE device keys could sometimes not be sent to remote servers (#2466)

* Fix flakey sytest 'Local device key changes get to remote servers'

* Debug logs

* Remove internal/test and use /test only

Remove a lot of ancient code too.

* Use FederationRoomserverAPI in more places

* Use more interfaces in federationapi; begin adding regression test

* Linting

* Add regression test

* Unbreak tests

* ALL THE LOGS

* Fix a race condition which could cause events to not be sent to servers

If a new room event which rewrites state arrives, we remove all joined hosts
then re-calculate them. This wasn't done in a transaction so for a brief period
we would have no joined hosts. During this interim, key change events which arrive
would not be sent to destination servers. This would sporadically fail on sytest.

* Unbreak new tests

* Linting

cmd/generate-keys/main.go

@@ -20,7 +20,7 @@ import (
 	"log"
 	"os"
 
-	"github.com/matrix-org/dendrite/internal/test"
+	"github.com/matrix-org/dendrite/test"
 )
 
 const usage = `Usage: %s