Applications can use the index to detect replays of the same message.

Some clients expect the session id to be globally unique,
so allowing the end devices to pick the session id will cause
problems.

Include the current ratchet index with the initial keys, this decreases
the risk that the client will supply the wrong index causing problems.

Sign the initial keys with the ratchet ed25519 key, this reduces the
risk of a client claiming a session that they didn't create.

Add ed25519 keys to the inbound and outbound sessions, and use them to sign and
verify megolm messages.

We just stuff the ed25519 public key in alongside the megolm session key (and
add a version byte), to save adding more boilerplate to the JS/python/etc
layers.

Includes creation of inbound sessions, etc