Take control of recovered snapshots, start restoration asynchronously (#2010)

* take control of given snapshot

* start snapshot restoration asynchronously,

ethcore/src/snapshot/tests/blocks.rs

@@ -79,7 +79,6 @@ fn chunk_and_restore(amount: u64) {
 	}
 
 	rebuilder.glue_chunks();
-	drop(rebuilder);
 
 	// and test it.
 	let new_chain = BlockChain::new(Default::default(), &genesis, new_db);

ethcore/src/snapshot/tests/state.rs

@@ -72,8 +72,9 @@ fn snap_and_restore() {
 			rebuilder.feed(&chunk).unwrap();
 		}
 
-		rebuilder.check_missing().unwrap();
 		assert_eq!(rebuilder.state_root(), state_root);
+		rebuilder.check_missing().unwrap();
+
 		new_db
 	};