## Problem
We want to define the algorithm for safekeeper membership change.
## Summary of changes
Add spec for it, several models and logs of checking them.
ref https://github.com/neondatabase/neon/issues/8699
The spec was written for the buggy protocol which we had before the one
more similar to Raft was implemented. Update the spec with what we
currently have.
ref https://github.com/neondatabase/neon/issues/8699