Background relay sync

[Sjors]

For a self hosted instance it would be nice if I can run a cron job that tracks relays for relevant stuff and serves it to the frontend as soon as it comes back online.

Something similar could be paid feature for users of the iris.to instance.

Not sure what the right implementation approach would be. One way would be to have it mimic a relayer in terms of interface, with the only difference being that it only has relevant content.

I also don't know if this can be done using only the public key. Using the nsec might be acceptable for a self-hosted solution, though still not ideal.