Make ChildJobs and friends picklable
Pave the way toward supporting the 'spawn' method of creating jobs, by adding support for pickling ChildJobs. Introduce a new 'jobpickler' module that provides an entrypoint for this functionality.
This also makes replays of jobs possible, which has made the debugging of plugins much easier for me.
This works towards #1021, BuildStream on win32.
Edited by Angelos Evripiotis