Multithreaded snapshot creation (#9239)

* Add Progress to Snapshot Secondary chunks creation

* Use half of CPUs to multithread snapshot creation

* Use env var to define number of threads

* info to debug logs

* Add Snapshot threads as CLI option

* Randomize chunks per thread

* Remove randomness, add debugging

* Add warning

* Add tracing

* Use parity-common fix seek branch

* Fix log

* Fix tests

* Fix tests

* PR Grumbles

* PR Grumble II

* Update Cargo.lock

* PR Grumbles

* Default snapshot threads to half number of CPUs

* Fix default snapshot threads // min 1

parity/cli/mod.rs

@@ -874,6 +874,10 @@ usage! {
 			"--no-periodic-snapshot",
 			"Disable automated snapshots which usually occur once every 10000 blocks.",
 
+			ARG arg_snapshot_threads: (Option<usize>) = None, or |c: &Config| c.snapshots.as_ref()?.processing_threads,
+			"--snapshot-threads=[NUM]",
+			"Enables multiple threads for snapshots creation.",
+
 		["Whisper Options"]
 			FLAG flag_whisper: (bool) = false, or |c: &Config| c.whisper.as_ref()?.enabled,
 			"--whisper",
@@ -1355,6 +1359,7 @@ struct Footprint {
 #[serde(deny_unknown_fields)]
 struct Snapshots {
 	disable_periodic: Option<bool>,
+	processing_threads: Option<usize>,
 }
 
 #[derive(Default, Debug, PartialEq, Deserialize)]
@@ -1788,6 +1793,7 @@ mod tests {
 			arg_export_state_at: "latest".into(),
 			arg_snapshot_at: "latest".into(),
 			flag_no_periodic_snapshot: false,
+			arg_snapshot_threads: None,
 
 			// -- Light options.
 			arg_on_demand_retry_count: Some(15),
@@ -2046,6 +2052,7 @@ mod tests {
 			}),
 			snapshots: Some(Snapshots {
 				disable_periodic: Some(true),
+				processing_threads: None,
 			}),
 			misc: Some(Misc {
 				logging: Some("own_tx=trace".into()),