Cache non-encrypted rooms and skip fetching encrypted rooms on startup

[Half-Shot]

We need this for specific situations where a bot is joined to a LOT of rooms, but the encrypted rooms are in the minority.

This should:

• Cache rooms that explicitly do not have a encryption state event.
• Recheck every time we see an encryption state event.

Checklist

• Tests written for all new code
• Linter has been satisfied
• Sign-off given on the changes (see CONTRIBUTING.md)

[AndrewFerr]

Looks like this introduces the possibility of the RoomTracker of an appservice intent's crypto client not doing a room check (via queueRoomCheck) on a known-encrypted room.

I believe the only consequence of that would be the bot-sdk's crypto store being slightly out-of-date until something else triggers a room check (and plenty of actions do trigger such a check). It shouldn't prevent events from being decryptable, however.

What would fix it is calling CryptoClient#isRoomEncrypted before each call to CryptoClient#decryptRoomEvent in Appservice.handleTransaction.

[Half-Shot]

What would fix it is calling CryptoClient#isRoomEncrypted before each call to CryptoClient#decryptRoomEvent in Appservice.handleTransaction.

So the only place we don't do this is in appservice, the MatrixClient always does a check first. I'll update that in my appservice PR.