update safekeepers internal docs

[stepashka]

review the overview about teh safekeepers, update, change names https://github.com/zenithdb/zenith/blob/main/walkeeper/README review and update the protocol too https://github.com/zenithdb/zenith/blob/main/walkeeper/README_PROTO.md

[banks]

Hey this project looks super cool!

I hope this is a useful place to note this but a few references still exist in the internal docs for safekeeper/README_PROTO.md (also linked in this issue) which is no longer in the source tree.

I'll clone the repo and go find it in history for now!

[hlinnaka]

Hey this project looks super cool!

I hope this is a useful place to note this but a few references still exist in the internal docs for safekeeper/README_PROTO.md (also linked in this issue) which is no longer in the source tree.

Thanks! I opened PR #2188 to fix the references, and while at it, I turned them into links.

[aome510]

Fix in #2188

[problame]

Arthur will put together an outline of what he wants to write, then post it to Slack for feedback.