core, eth: abort snapshot generation on snap sync and resume later #22777
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The new snap sync algo reuses the snapshot namespace instead of wiping and regenerating at the end. This means we need to disable snapshots from being used during sync. This disabling needs to survive restarts too.
A previous attempt tried to just nuke the journal, but that didn't really do anything as the existing snapshot was already live and used and a restart just considered everything missing and started a regeneration cycle from scratch, deleting the valuable partial data.
This PR solves it by adding a new marker to the database to track disabled snapshots, and on top it also aborts any active generation on demand and marks all existing layers stale so any accidental live use gets gracefully stopped too.