Support m.room_key events and the related creation of Megolm sesisons

This brings us very close towards being able to decrypt messages that were sent using Megolm.

This MR does the following:

• Extend the session cache to support the caching of Megolm sessions
• check olm encrypted events for m.room_key events when decrypting successfully
• search for a session to avoid duplicates and the creation of a new one if previously non-existent

The relevant section in the E2E impl. guide is here: https://matrix.org/docs/guides/e2e_implementation.html#handling-an-room-key-event