Adds a very limited ability to try and look for false positive /
non-determinstic staging errors. It tries to err on the side of
limiting false false positives, so it's likely to miss many.
Currently has no automation / reporting, just sets a flag on the
stagings which are strongly believed to have failed due to false
positives.
While at it, add link between a "root" staging and its splits. It's
necessary to clear the "false positive" flag, and surfacing it in the
UI could be useful one day.
Fixes#660
