The syncserver now asks the roomserver via QueryBackfill (which already
existed to *handle* backfill requests) which then makes federation requests
via gomatrixserverlib.RequestBackfill.
Currently, tests fail on subsequent /messages requests because we don't know
which servers are in the room, because we are unable to get state snapshots
from a backfilled event because that code doesn't exist yet.
* Experimental LRU cache for room versions
* Don't accidentally try to type-assert nil
* Also reduce hits on query API
* Use hashicorp implementation which mutexes for us
* Define const for max cache entries
* Rename to be specifically immutable, panic if we try to mutate a cache entry
* Review comments
* Remove nil guards, give roomserver integration test a cache
* go mod tidy
