Start a new megolm session when devices are blacklisted
If we have shared the session with a device which is subsequently blacklisted,
we need to start a new session for the next message.
Rather than doing this proactively (which would be subject to false-positives
and require slightly awkward tracking of who we had shared the session with),
we check the list of who we have shared the session with on each send, and
start a new session if any of them are blocked.
Fixes https://github.com/vector-im/riot-web/issues/2146.
