Expire/refresh old Objects

[snarfed]

Would be nice to have a way to specify how long Objects are valid before we need to refresh them. Probably per-protocol, and even may vary with protocols, eg ATProto commits and CID-based objects are permanent, records vary, and did:plc resolution is currently 24-72h ish (bluesky-social/atproto#1535). Also, if refresh fails, we can probably often use our stale stored version, but maybe not always?

[snarfed]

Couple options here:

• Datastore TTL policy, like we already do to expire transient activities like creates, updates, deletes.
• Age limit in Protocol.load after which we refetch.

Leaning toward the latter so that we preserve older metadata like Object.created.

[snarfed]

Did the latter. 🤞