Use a lock instead of atomics for snapshot Progress (#11197)

* WIP. Typos and logging.

* Format todos

* Pause pruning while a snapshot is under way
Logs, docs and todos

* Allocate memory for the full chunk

* Name snapshotting threads

* Ensure `taking_snapshot` is set to false whenever and however `take_snapshot`returns
Rename `take_at` to `request_snapshot_at`
Cleanup

* Let "in_progress" deletion fail
Fix tests

* Just use an atomic

* Review grumbles

* Finish the sentence

* Resolve a few todos and clarify comments.

* Calculate progress rate since last update

* Lockfile

* Fix tests

* typo

* Reinstate default snapshotting frequency
Cut down on the logging noise

* Use a lock instead of atomics for snapshot Progress

* Update ethcore/types/src/snapshot.rs

Co-Authored-By: Andronik Ordian <write@reusable.software>

* Avoid truncating cast
Cleanup

ethcore/src/client/client.rs

@@ -2543,7 +2543,7 @@ impl SnapshotClient for Client {
 		&self,
 		writer: W,
 		at: BlockId,
-		p: &Progress,
+		p: &RwLock<Progress>,
 	) -> Result<(), EthcoreError> {
 		if let Snapshotting::Unsupported = self.engine.snapshot_mode() {
 			return Err(EthcoreError::Snapshot(SnapshotError::SnapshotsUnsupported));