Add cursor hint to allow syncing low activity PDSs

[derrandz]

Closes #1064

This PR implement a cursor hint in requestCrawl.

From the issue

If the relay hasn't seen the host before, it uses that to scroll back a tiny bit (decrements the cursor), which should result in an event being received, and thus the seq getting recorded.

This PR also adds a new configuration key CursorHintScrollBack

Tests

$ go test -v -run 'TestCursorHint' ./cmd/relay/testing

Next steps

Must updated PDS implementation(s) to use the new lexicon for requestCrawl

[bnewbold]

This is interesting, but we should discuss it as a protocol change before doing any implementation work.

Could you open a discussion in github discussions (https://github.com/bluesky-social/atproto/discussions) or on discourse (https://discourse.atprotocol.community/)?

we also have some unrelated relay operational changes in the works, so it might be a while before we can dig in to this.

[derrandz]

Opened a discussion at bluesky-social/atproto#4595