Skip room join until we run into a 403/401

[MadLittleMods]

Spawned from #31 (comment)

---

Most of the time, we're already joined to the room so there is no need to try joining the room every single time a request is made.

We can instead skip the join and go straight into fetching the room metadata. If we encounter a403 Forbidden or 401 Unauthorized, we can then run the join and run the data fetchers again.

[MadLittleMods]

Exploring this while addressing #107 because we need to get the roomId for a room alias to use with the various Matrix API's and /join/{roomIdOrAlias} -> { room_id } is a great way to get it. It also solves the federation problem for rooms the server doesn't already know about

The only alternative is to use GET /_matrix/client/r0/directory/room/{roomAlias} -> { room_id, servers }.

Time-wise, the two endpoints aren't too different and /join may even be slower. But complexity-wise, it's nice to always go down the same code-path regardless. If anything, we can improve the subsequent join time in Synapse.

Comparison timing

POST /_matrix/client/v3/join/{roomIdOrAlias} 
1289ms
277ms
435ms
715ms
476ms
320ms
619ms
374ms
684ms
419ms
400ms
270ms

GET /_matrix/client/r0/directory/room/%23matrix%3Amatrix.org
240ms
578ms
314ms
237ms
241ms
243ms
242ms
251ms
387ms
232ms
238ms
250ms

I guess we will close this for now since getting the room ID from /join seems best.