`get_room_version` doesn't use the cache

[richvdh]

we have two functions for getting the version of a room from the database, get_room_version_id and get_room_version. The former gets the raw string, and then caches it to speed up future lookups. The second looks up the raw string, and then consults the list of known room versions to decode it into a RoomVersion object. The problem is that it doesn't cache the result, so is generally slower than it should be.

If get_room_version needs to exist at all, it should probably just wrap get_room_version_id.

[richvdh]

seems to be a regression introduced in #10245

[lukasdenk]

I'll work on it