Origin/Cache downtime ops should not rely on Registry response

[h2zh]

Pelican Service:

• Client
• Plugin
• Registry
• Director
• Origin
• Cache
• Other (please give the detail)

Is your feature request related to a problem? What is the proposed solution? Please describe.
This was spun out from a review comment on PR #2785 to be addressed separately.

The Origin/Cache should not rely on a response from the Registry to populate its own downtime table. We decided we'd trade "immediately consistent" for "eventually consistent" when it comes to potential skew between what the Origin/Cache knows about its downtime and what the Registry/Director knows about the downtime.

Also, we need to figure out how to merge two potentially-conflicting downtimes when one comes from the Origin/Cache and the other comes from the Registry. [original request]