In what I hoped would be a five-minute refactor to help clean up an annoying
not-really-used codepath, but turned into a bit of a hackathon on the tests,
create Olm sessions lazily in Olm rooms, just as we do in megolm rooms, which
allows us to avoid having to get the member list before configuring e2e in a
room.
It is possible for `room` to be null when passed to
MegolmEncryption.encryptMessage; we need to avoid dereferencing it. Instead,
make sure that the EncryptionAlgorithm knows about the roomId it is targeting,
and use that.
Replace the increasingly-long argument list on the EncryptionAlgorithm
constructor with a params list, and update DecryptionAlgorithm to match.
- 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.
