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.
