E2E | Sending megolm keys sooner

Refactor key sending to operate as follows:
- Get olm sessions that are already established
- In parallel:
- Send megolm keys to existing olm sessions
- Start olm sessions with new devices, but with a 2s timeout to /keys/claim
- Send org.matrix.room_key.withheld (MSC2399) messages to blocked devices
- Send message to room
- Start olm sessions with devices that it was unable to establish a session with earlier, but with no timeout specified to /keys/claim (which will result in a 10s timeout)

When a user starts typing in an encrypted room, start creating olm sessions and sending megolm keys (as above), so that fewer keys need to be sent when the user hits “send”. (In web, we do this every time the user hits a key, up to a maximum of once a minute.)