Parameter to allow user to force the sealing mechanism (#918)

* Allow block sealing mechanism to be forced, even when not mining.

* Fix deadlock in dispatch_transaction. Fix tests.

* Horrible workaround for transaction importing.

* Reduce tracing. Cleanups.

* Remove logging.

* Remove broken code inherited from dodgy implementation.

* pre-query tx queue nonce also if any

* remove outside nonce queries

* remove queue nonces

sync/src/tests/helpers.rs

@@ -93,7 +93,7 @@ impl TestNet {
 		for _ in 0..n {
 			net.peers.push(TestPeer {
 				chain: TestBlockChainClient::new(),
-				sync: ChainSync::new(SyncConfig::default(), Miner::new()),
+				sync: ChainSync::new(SyncConfig::default(), Miner::new(false)),
 				queue: VecDeque::new(),
 			});
 		}