Make 'mop' suitable for cron job

[gsaksena]

Running mop may disrupt workflows that are currently running, so today one needs to ensure everything is quiet before running it.

While this is not an issue for workspaces you want to mothball, it means you cannot run it asynchronously as part of a cron job (or Google Cloud Scheduler job).

One way to do that might be to only delete objects created older than a certain age. Eg as tasks are limited to 7 days, and people tend to not like really long workflows, limiting the purge to objects created > 10 days ago will likely avoid clobbering a running pipeline. If the user knows the pipelines are all much shorter, a shorter timeframe could be selected. A longer timeframe, eg 60 days, might be chosen if they want to ensure call caching on their current project would not be impacted.

Tighter coupling between mop and the Cromwell/Terra databases might offer a cleaner solution, but if that's too complicated the easier solution above would still be an improvement.