Backport of:
- https://github.com/sigp/lighthouse/pull/7067
For:
- https://github.com/sigp/lighthouse/issues/7039
- Prevent writing to state cache when migrating the database
- Add `state-cache-headroom` flag to control pruning
- Prune old epoch boundary states ahead of mid-epoch states
- Never prune head block's state
- Avoid caching ancestor states unless they are on an epoch boundary
- Log when states enter/exit the cache
Co-authored-by: Eitan Seri-Levi <eserilev@ucsc.edu>