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

ethcore/src/tests/helpers.rs

@@ -193,7 +193,8 @@ pub fn generate_dummy_client_with_spec_accounts_and_data<F>(get_test_spec: F, ac
 			Arc::new(last_hashes.clone()),
 			author.clone(),
 			(3141562.into(), 31415620.into()),
-			vec![]
+			vec![],
+			false,
 		).unwrap();
 		b.set_difficulty(U256::from(0x20000));
 		rolling_timestamp += 10;