Retrospective region meta update reworked

Here's my crack at reworking #116. I think the thing that was causing most of the additional complexity was the whole "job" concept on top of the WordPress cron.

In 47d6dbaf5409b18ae65923b0b272e6a780152d37 I've gotten rid of that and I think the code is quite a bit simpler and easier to understand, so should be less prone to error.

I recommend going through each commit individually rather than look at the PR diff. Will be easier to understand.