tree/request_impl: more improvements
tree/request_impl: more improvements

1. Instead of sing request->response idiom on worker actor to obtain
   request result for extra processing, pass custom result
   processing callback with `a_ack` message. Then it will be invoked
   right inside worker actor w/o additional request.

2. Modify `request_data*()` helpers s.t. they support invoke without
   extra result processor. In that case handy `caf::result` value is
   returned.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re