* Add a way to signal shutdown to snapshotting threads
* Pass Progress to fat_rlps() so we can abort from there too.
* Checking for abort in a single spot
* Remove nightly-only weak/strong counts
* fix warning
* Fix tests
* Add dummy impl to abort snapshots
* Add another dummy impl for TestSnapshotService
* Remove debugging code
* Return error instead of the odd Ok(())
Switch to AtomicU64
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