Fix performance regression in `get_users_in_room` (#13972)

Fixes #13942. Introduced in #13575.

Basically, let's only get the ordered set of hosts out of the DB if we need an ordered set of hosts. Since we split the function up the caching won't be as good, but I think it will still be fine as e.g. multiple backfill requests for the same room will hit the cache.