Race condition with room creation means room directory lacks read-after-write consistency in monolith mode

[MadLittleMods]

It's possible to query the room directory after the room creation and not see the room.

Reproduction steps

1. Using monolith Synapse
2. Create a public room called foo with /createRoom
3. Immediately after seeing a successful create room response, search the room directory for the room (/publicRooms with { filter: { generic_search_term: 'foo' } })
   • It's important to use search because it bypasses the cache
4. Notice that the room is missing from the response ❌

If you give the homeserver some time, the room will show up.

Notes

This regressed in #15229 where I guess the timing changed just enough for this to be possible. You can see the full bisect, investigation and context for how I noticed in matrix-org/matrix-viewer#208 (comment)

It seems like we're await'ing everywhere so not sure why this would happen. And seems weird that it would happen in monolith mode since there is not replication lag and we're awaiting everything.