Keep existing blocks when restoring a Snapshot (#8643)

* Rename db_restore => client

* First step: make it compile!

* Second step: working implementation!

* Refactoring

* Fix tests

* PR Grumbles

* PR Grumbles WIP

* Migrate ancient blocks interating backward

* Early return in block migration if snapshot is aborted

* Remove RwLock getter (PR Grumble I)

* Remove dependency on `Client`: only used Traits

* Add test for recovering aborted snapshot recovery

* Add test for migrating old blocks

* Fix build

* PR Grumble I

* PR Grumble II

* PR Grumble III

* PR Grumble IV

* PR Grumble V

* PR Grumble VI

* Fix one test

* Fix test

* PR Grumble

* PR Grumbles

* PR Grumbles II

* Fix tests

* Release RwLock earlier

* Revert Cargo.lock

* Update _update ancient block_ logic: set local in `commit`

* Update typo in ethcore/src/snapshot/service.rs

Co-Authored-By: ngotchac <ngotchac@gmail.com>

ethcore/service/src/service.rs

@@ -117,7 +117,7 @@ impl ClientService {
 			pruning: pruning,
 			channel: io_service.channel(),
 			snapshot_root: snapshot_path.into(),
-			db_restore: client.clone(),
+			client: client.clone(),
 		};
 		let snapshot = Arc::new(SnapshotService::new(snapshot_params)?);