db: limit EFOS sstable pinning to files overlapping bounds

Eventually file-only snapshots suffer additional space amplification by pinning a readState/`manifest.Version` until closed. This prevents any sstables in the version from being deleted. Since an EFOS indicates the bounds that it requires consistency over, we can avoid pinning sstables outside those bounds.

Jira issue: PEBBLE-100

Start by tracing file-only EFOS snapshot handling around readState and manifest.Version, then inspect how sstables are pinned until the snapshot closes. Done means an EFOS pins only sstables whose ranges overlap its consistency bounds, while files outside those bounds can be deleted.

Written by the indexing model from the issue text.