Don't block sync when importing old blocks (#8530)

* Alter IO queueing.

* Don't require IoMessages to be Clone

* Ancient blocks imported via IoChannel.

* Get rid of private transactions io message.

* Get rid of deadlock and fix disconnected handler.

* Revert to old disconnect condition.

* Fix tests.

* Fix deadlock.

ethcore/sync/src/tests/helpers.rs

@@ -520,11 +520,9 @@ impl TestIoHandler {
 impl IoHandler<ClientIoMessage> for TestIoHandler {
 	fn message(&self, _io: &IoContext<ClientIoMessage>, net_message: &ClientIoMessage) {
 		match *net_message {
-			ClientIoMessage::NewMessage(ref message) => if let Err(e) = self.client.engine().handle_message(message) {
-				panic!("Invalid message received: {}", e);
-			},
-			ClientIoMessage::NewPrivateTransaction => {
+			ClientIoMessage::Execute(ref exec) => {
 				*self.private_tx_queued.lock() += 1;
+				(*exec.0)(&self.client);
 			},
 			_ => {} // ignore other messages
 		}