origin visit reaper: new janitorial process in charge of updating lingering origin visit in 'ongoing' state

We have a somewhat common (but fairly infrequent) pattern of visits crashing hard, and lingering forever in an ongoing state.

Adding a new component "origin visit reaper" in charge of updating those lingering origin visits sounds the appropriate way to deal with those.

If we allow noop state transitions, we can use them as a heartbeat process. We can then specify that ongoing visits with no state update in the last $timeframe are dead, and should be set as failed by an external process. This process would not depend on the worker scaffolding, but just be a janitorial process that runs once in a while to keep the archive tidy.

Original idea appeared in T2310#43199 discussion.

---

Migrated from T2372 (view on Phabricator)