Get event/notification sending working correctly

In order to support tasks notifications and progress
updates we need to establish a channel & proxy by which
those events can be sent from the process executing and
producing those events and the originating process that
requested that task to be executed.

This review adds on such a proxy and adjusts a cloned tasks
notification callbacks to place messages on a queue that will
be picked up by a thread in the originating process for dispatch
to the original callbacks that were registered with the non-cloned
task (therefore making the original callbacks appear to be called
as they are supposed to be).

Part of blueprint process-executor

Change-Id: I01c83f13186e4be9fa28c32e34e907bb133e8fb3
Merge "Get event/notification sending working correctly"