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.
