[Buildstream 1.4] Add support for Python 3.8

ChildWatcher.add_child_handler needs to be called from a main loop and is dispatched through that main loop.

To be able to properly dispatched the ChildWatcher handlers, Scheduler._terminate_jobs_real had to be made loop friendly by yielding between termination and kill so that it knows which process have been terminated.

Since Scheduler._terminate_jobs_real now yields instead of blocking, we need to avoid calling it multiple times from the main loop.