FInd more efficient way to check room memberships for appservices

*This issue was originally created by [**@anoadragon453**](https://github.com/anoadragon453) at .*

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.