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