react immediately on PLS SYNCH without running job
Only stop commenter when no relevant results could follow

If we have no pending jobs, `overruleSuccess` returns no results,
and the commenter can stop.

Also improved logging a bit.
Urgh. Off by one negation.
Don't know when to kill PR commenter; keep it alive

The job start watcher may call at any time as a result
of a PLS SYNCH or a PLS REBUILD ALL, so we can't stop the commenter
when the PR's status seems definitive, as we may want to force-update
the other statuses regardless.

Thus, just keep it alive as long as its parent is.
TODO: figure out how all these actors relate and make sure
to stop them when no longer needed
final cleanup: only report on most recent build
start watcher should not retry PLS SYNCH

There was a logic flaw in the retry condition:
`b.noop` implies `(currentBuilds.toSet diff prehistoricBuilds).isEmpty`,
so it's not a good stop criterion -- use `reportedBuilds` instead.
Merge pull request #47 from adriaanm/no-premature-stop

No premature stop