RoomHierarchyRestServlet hammers the DB way too hard

[ara4n]

Looking at the resource graphs for the main synapse process on attendees.fosdem.org, by far the largest resources (by 10x) are being consumed by RoomHierarchyRestServlet, which is hammering away at get_room_version at ~75Hz, and averaging 150ms of db time per transaction:

Feels like we're missing a cache, given the room hierarchy is hardly going to be changing much, and the room version even less(!)

[clokep]

Looking at the resource graphs for the main synapse process on attendees.fosdem.org

Note that this API is safe to go to workers.

Feels like we're missing a cache, given the room hierarchy is hardly going to be changing much

The hierarchy depends on the requester, so is a bit hard to cache efficiently.

and the room version even less(!)

This is #11706 and is a regression. There's a fix at #11808.

[clokep]

Maybe related: #11694.

[clokep]

I think this was essentially a duplicate of #11706 and not specifically related to the hierarchy endpoint.