Two main changes here:
* when we get an m.new_device event for a device we know about, ignore it
* Batch up the m.new_device events received during initialsync and spam out
all the queries at once.
In order to mitigate backward-secrecy concerns, make sure that we rotate the
outbound megolm session at regular intervals (every week/100 msgs by default).
Finish plumbing in the Ed25519 key checks. Make sure we store the claimed key
correctly in the megolm sessions, and keep them as a separate field in
MatrixEvent rather than stuffing them into _clearEvent
