DM: handle recipient list with remote recipients

Handle new recipient list format in which the MESSAGE request has all recipients in it, not just ones local to the responder.

See also:

https://github.com/Libertree/libertree/pull/8
https://github.com/Libertree/libertree-model-rb/pull/5
https://github.com/Libertree/libertree-client-rb/pull/2
https://github.com/Libertree/libertree-frontend-ramaze/pull/31