(C2M) run C2M "continuously" against the least recently checked catalog entries

[jmartin-sul]

questions:

• what's the mechanism? cron job via whenever gem?
• should we add a lock file (or similar, e.g. flag in the DB) indicating that C2M check_existence is already running, so that the cron jobs don't step on one another and pick up the same list of least-recently-checked objects if they run long?

[ndushay]

current plan, in #648, is to run every other week via cron (whenever).

after that, we may do any of the following:

• run all the roots in parallell, effectively starting a job that runs the rake task for a single storage root ... for each of the storage roots. (one process per root)
  • determine the longest time needed for any storage root, and possibly run cron every XXX depending on that interval.
• switch to using redis to kick off C2M for individual druids

[jmartin-sul]

implemented per #648, then re-worked to use queues in #1062