Make InstantSeal Instant again (#11186)

* Make InstantSeal Instant again

* update_sealing if there are transactions in pool after impoerting a block, some line formatting

* Apply suggestions from code review

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* InstantSeal specific behaviour

* introduce engine.should_reseal_on_update, remove InstantSealService

* remove unused code

* add force param to update_sealing

* better docc

* even better docs

* revert code changes, doc corrections, sort dep

* code optimization

* fix test

* fix bench

ethcore/engine/src/engine.rs

@@ -188,6 +188,14 @@ pub trait Engine: Sync + Send {
 	/// Returns the engine's current sealing state.
 	fn sealing_state(&self) -> SealingState { SealingState::External }
 
+	/// Called in `miner.chain_new_blocks` if the engine wishes to `update_sealing`
+	/// after a block was recently sealed.
+	///
+	/// returns false by default
+	fn should_reseal_on_update(&self) -> bool {
+		false
+	}
+
 	/// Attempt to seal the block internally.
 	///
 	/// If `Some` is returned, then you get a valid seal.