More performance optimizations (#1814)

* Buffered DB

* Use identity hash for MemoryDB

* Various tweaks

* Delayed DB compression

* Reduce last_hashes cloning

* Keep state cache

* Updating tests

* Optimized to_big_int

* Fixing build with stable

* Safer code

ethcore/src/client/test_client.rs

@@ -272,7 +272,7 @@ impl MiningBlockChainClient for TestBlockChainClient {
 			false,
 			db,
 			&genesis_header,
-			last_hashes,
+			Arc::new(last_hashes),
 			author,
 			gas_range_target,
 			extra_data