Log state of Olm sessions
...whenever we encrypt or decrypt a message on them. This adds
another line of logging for every device in the room, so will
be reasonably verbose if you're in large encrypted rooms, but
the information ought to be valuable.
Requires https://gitlab.matrix.org/matrix-org/olm/merge_requests/9
Don't merge before a new version of Olm is released with this merge
request (it won't work).
