bug 845918: Switch to multi-process work queue for evaluating macros

Okay, I think this is finally at a point where it can be considered as a PR.

Probably best to *not* squash the multiple commits: Each has turned out to be a decent-sized change on its own, and it might help to review them one at a time rather than the entire diff in one shot.

To spot-check: Try this updated kumascript service in kuma, and the only thing you should notice (if anything) is that things might be a bit faster and errors might be a bit more detailed.