PoA: Wait for transition finality before applying (#5774)

* final engine changes

* migration to v13

* adding and removing pending transitions

* epoch_transition_for

* port snapshot to new engine methods

* final validator set interface

* fix compiler errors

* revert v13/epoch_depth transition

* make call on new epoch

* rolling finality checker

* tests for finality checker

* constructing finality proof upon pending transition

* fix warnings and finality proof checking

* fix compiler warnings in tests

* test fixes

* don't include genesis in finality checking

* change snapshot test chain building logic

* minor refactorings

* fetch epoch transition based on parent, fix divide-by-zero in SimpleList

* fix formatting

* fix ABIs and finality checking in snapshot restoration

* encode signal number in proof

* create more blocks at the end of tests

* update gist to accurate contract code

* test for epoch_transition_for

* fix tests with immediateTransitions parameter

* disable force flag after forcing

* rename ValidatorsChanged to InitiateChange and finalizeSignal to finalizeChange

* a few more validator set tests

json/src/spec/authority_round.rs

@@ -49,6 +49,9 @@ pub struct AuthorityRoundParams {
 	/// Block from which monotonic steps start.
 	#[serde(rename="validateStepTransition")]
 	pub validate_step_transition: Option<Uint>,
+	/// Whether transitions should be immediate.
+	#[serde(rename="immediateTransitions")]
+	pub immediate_transitions: Option<bool>,
 }
 
 /// Authority engine deserialization.
@@ -92,5 +95,6 @@ mod tests {
 		assert!(deserialized.params.registrar.is_none());
 		assert_eq!(deserialized.params.start_step, Some(Uint(U256::from(24))));
 		assert_eq!(deserialized.params.eip155_transition, Some(Uint(U256::from(0x42))));
+		assert_eq!(deserialized.params.immediate_transitions, None);
 	}
 }