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/private-tx/tests/private_contract.rs

@@ -74,7 +74,7 @@ fn private_contract() {
 			Box::new(NoopEncryptor::default()),
 			config,
 			io,
-	).unwrap());
+	));
 
 	let (address, _) = contract_address(CreateContractAddress::FromSenderAndNonce, &key1.address(), &0.into(), &[]);