Expanse Byzantium update w/ correct metropolis difficulty increment divisor (#7463)

* Byzantium Update for Expanse

Here the changes go. Hope I didnt miss anything.

* expip2 changes - update duration limit

* Fix missing EXPIP-2 fields

* Format numbers as hex

* Fix compilation errors

* Group expanse chain spec fields together

* Set metropolisDifficultyIncrementDivisor for Expanse

* Revert #7437

* Add Expanse block 900_000 hash checkpoint

ethcore/src/tests/helpers.rs

@@ -385,5 +385,7 @@ pub fn get_default_ethash_params() -> EthashParams {
 		eip649_transition: u64::max_value(),
 		eip649_delay: 3_000_000,
 		eip649_reward: None,
+		expip2_transition: u64::max_value(),
+		expip2_duration_limit: 30,
 	}
 }