Memory-based pruning history size (#4114)

* prune states based on memory param

* pruning memory CLI and usage in sync

* return purged value from memorydb

* calculate memory used incrementally in overlayrecentdb

* refactor shared history pruning code in client

* Fixed usage alignment

* journal_size function for fast memory calculation

ethcore/src/client/test_client.rs

@@ -714,10 +714,10 @@ impl BlockChainClient for TestBlockChainClient {
 	fn disable(&self) { unimplemented!(); }
 
 	fn pruning_info(&self) -> PruningInfo {
+		let best_num = self.chain_info().best_block_number;
 		PruningInfo {
 			earliest_chain: 1,
-			earliest_state: 1,
-			state_history_size: *self.history.read(),
+			earliest_state: self.history.read().as_ref().map(|x| best_num - x).unwrap_or(0),
 		}
 	}