New recipient subhash on MESSAGE requests:

 include public_key of recipient's tree.

See also:

https://github.com/Libertree/libertree-model-rb/pull/5
https://github.com/Libertree/libertree-client-rb/pull/2
https://github.com/Libertree/libertree-backend-rb/pull/5
https://github.com/Libertree/libertree-frontend-ramaze/pull/31