Add a way to signal shutdown to snapshotting threads (#10744)

* 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

rpc/src/v1/tests/helpers/snapshot_service.rs

@@ -48,6 +48,7 @@ impl SnapshotService for TestSnapshotService {
 	fn status(&self) -> RestorationStatus { self.status.lock().clone() }
 	fn begin_restore(&self, _manifest: ManifestData) { }
 	fn abort_restore(&self) { }
+	fn abort_snapshot(&self) {}
 	fn restore_state_chunk(&self, _hash: H256, _chunk: Bytes) { }
 	fn restore_block_chunk(&self, _hash: H256, _chunk: Bytes) { }
 	fn shutdown(&self) { }