Remote transaction execution (#4684)

* return errors on database corruption

* fix tests, json tests

* fix remainder of build

* buffer flow -> request credits

* proving state backend

* generate transaction proofs from provider

* network messages for transaction proof

* transaction proof test

* test for transaction proof message

* fix call bug

* request transaction proofs from on_demand

* most of proved_execution rpc

* proved execution future

ethcore/src/client/test_client.rs

@@ -765,6 +765,10 @@ impl ProvingBlockChainClient for TestBlockChainClient {
 	fn code_by_hash(&self, _: H256, _: BlockId) -> Bytes {
 		Vec::new()
 	}
+
+	fn prove_transaction(&self, _: SignedTransaction, _: BlockId) -> Option<Vec<DBValue>> {
+		None
+	}
 }
 
 impl EngineClient for TestBlockChainClient {