Port much of `synapse.handlers.federation` to async/await.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

I messed this up a bit in #6805, but fortunately we weren't actually doing
anything with the room_version so it didn't matter that it was a str not a RoomVersion.

We were in fact only deleting stale marker when we got an incremental
update, rather than when we did a full resync.

Pass room_version into add_hashes_and_signatures

Make `get_room_version` return a RoomVersion object