FInd more efficient way to check room memberships for appservices

[matrixbot]

This issue was originally created by @anoadragon453 at matrix-org/dendrite#468.

Currently app services will get events related to their namespace from any room on the homeserver, even if they are not part of that room yet. This isn't really intended behavior, and thus should be scoped to only the room the app service is a part of.

[matrixbot]

This comment was originally posted by @Half-Shot at matrix-org/dendrite#468 (comment).

I don't believe this is still true, do you think @neilalexander ?

[matrixbot]

This comment was originally posted by @neilalexander at matrix-org/dendrite#468 (comment).

We now have this as of #1680 but the process isn't amazing so we'd like to find a better/faster way to do it.