Ruma can't currently handle an event with the wrong event content type.
When replacing the MessageEventStub's content it automatically
serializes as "m.room.redaction".
The client objects in the sdk accepted an optional session argument at
creation time. If provided this would restore the client into a logged
in state.
Since we need to restore the client state and encryption state at some
point it makes sense to do it when the client transitions into a logged
in state. At the same time we would like to avoid the client creation to
be async.
This introduces a restore_login() method which can be called instead of
the login() method to restore a session.
