This PR fixes an issue where flat states are continuously persisted
during downloadState, while the sync journal is only persisted at the
end of Sync.
As a result, an unclean shutdown can leave the on-disk flat state ahead
of the journal markers. Some persisted entries may be stale (storage
slots that should have been deleted), and these dangling entries are not
detected or fixed by subsequent state downloads.
To address this, this PR introduces a cleanup step before state
downloading begins. It removes all state entries that are not covered by
the persisted journal markers.
7122ecc3eb