Tree: implements write() and set_write_eof()

Replace erroneous/passive code to fix write() and set_write_eof() on
WorkerTree instances. Use ControlMessage with new actions (write, eof)
to propagate these "actions" on PropagationChannel

WorkerTree.abort() is not implemented for now. In non-tree mode,
worker.abort() will stop all worker actions/events on the engine side.
This needs more work to be tree mode "safe" and have exactly the same
behavior in all cases.

Closes #206.

Change-Id: Ief988b99b0f112f5ec8411a1ec8b26484b142fac