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/tests/helpers.rs

@@ -160,7 +160,7 @@ pub fn generate_dummy_client_with_spec_and_data<F>(get_test_spec: F, block_numbe
 			false,
 			db,
 			&last_header,
-			last_hashes.clone(),
+			Arc::new(last_hashes.clone()),
 			author.clone(),
 			(3141562.into(), 31415620.into()),
 			vec![]