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>

rpc/src/v1/impls/parity.rs

@@ -440,7 +440,7 @@ impl<C, M, U, S> Parity for ParityClient<C, M, U> where
 			BlockNumber::Earliest => BlockId::Earliest,
 			BlockNumber::Latest => BlockId::Latest,
 		};
-		let receipts = try_bf!(self.client.block_receipts(id).ok_or_else(errors::unknown_block));
+		let receipts = try_bf!(self.client.localized_block_receipts(id).ok_or_else(errors::unknown_block));
 		Box::new(future::ok(receipts.into_iter().map(Into::into).collect()))
 	}