prhub 0.1.6

Fixed

• Polls no longer grow with every commit anyone ever asked about. A state or wait on a commit kept it tracked for
  6 hours, and each finished commit was re-read every 5 minutes, so watches piled up and came due together: seen on
  the maintainer's machine 15/10 with about 60 open PRs, 198 watched commits (170 not asked about for over an hour)
  made every fifth-minute poll read about 190 commits and take 7 to 13 minutes, and waits went that long without fresh
  data. Now a state keeps a commit tracked for 15 minutes, a running wait renews its claim every 5 minutes and
  lets it lapse 10 minutes after it ends, and a poll reads at most 50 commits: those never read or just asked about
  first, then those still running, then the stalest finished ones. Watches written by an older prhub expire an hour
  after they were last asked about, instead of six. Updates to commit-watch.json take a lock, and a commit GitHub
  keeps failing on is skipped for 5 minutes so it cannot crowd out the rest.