Early draft of caching

[Yoric]

This is a first attempt to add a minimal layer of caching on top of MatrixClient.

The objective is to reduce the amount of calls to the homeserver, plus gain performance. Some auditing shows that we place lots of calls that are simply useless because we are calling to request data that we already have. However, we don't want to cache everything as that would cost considerable amounts of memory in the Many-Mjölnir scenario. So we rather let code register for caching stuff that it is interested in.

In practice:

• we now cache the user id, localpart, domain in a single place (previously, it was cached in many places, uncached in many more);
• code can request a cache for specific account data types (previously, very little of it was cached);
• in the future, I'd like to expand this with the ability to request a cache for specific state events in specific rooms (at the moment, none of it is cached afaict).

Just caching the user id should already visibly reduce the amount of requests that we perform.