I've been trying to track down issues with the OlmSessions getting out of sync
between two devices. To help with this, add a method which can be used from the
JS console to inspect the state of OlmSessions.
A couple of small refactors which fell out of the aborted stuff for upgrading
to secure Ed25519 keys, but are useful in their own right.
The main functional change here is to calculate the "algorithms" list from
DECRYPTION_CLASSES (and hence include megolm in the list).
- to make way for alternative encryption algorithms. We now store an encryption
object for each room, rather than referring to sessionstore on each event.
Also a little light tidying to the jsdocs.
