feat(identity): long-lived Redis caching

[chris13524]

Milestone 3 from here: https://docs.google.com/document/d/1U2BxNHLb0YSveIxD8ix3UQYs_HZXQtZD7gfepMnUNTA/edit#heading=h.zfelgliclewt

Cache for as long as the ENS registration is valid for. Watch for blockchain events to know if cache should be invalidated sooner.

[chris13524]

Current consensus is that we don't need this yet, it's complicated and we haven't heard any complaints or had any problems with the current system.

Slack conversation