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/block_sync.rs

@@ -496,7 +496,7 @@ impl BlockDownloader {
 			}
 
 			let result = if let Some(receipts) = receipts {
-				io.chain().import_block_with_receipts(block, receipts)
+				io.chain().queue_ancient_block(block, receipts)
 			} else {
 				io.chain().import_block(block)
 			};