Benchmark Ethereum Pallet (#149)

* Add skeleton for worst case import_unsigned_header

* Fix a typo

* Add benchmark test for best case unsigned header import

* Add finality verification to worst case bench

* Move `insert_header()` from mock to test_utils

Allows the benchmarking code to use this without having to pull it in from the mock.

* Add a rough bench to test a finalizing a "long" chain

* Try to use complexity parameter for finality bench

* Improve long finality bench

* Remove stray dot file

* Remove old "worst" case bench

* Scribble some ideas down for pruning bench

* Prune headers during benchmarking

* Clean up some comments

* Make finality bench work for entire range of complexity parameter

* Place initialization code into a function

* Add bench for block finalization with caching

* First attempt at bench with receipts

* Try and trigger validator set change

* Perform a validator set change during benchmarking

* Move `validators_change_receipt()` to shared location

Allows unit tests and benchmarks to access the same helper function
and const

* Extract a test receipt root into a constant

* Clean up description of pruning bench

* Fix cache and pruning tests

* Remove unecessary `build_custom_header` usage

* Get rid of warnings

* Remove code duplication comment

I don't think its entirely worth it to split out so few lines of code.
The benches aren't particularly hard to read anyways.

* Increase the range of the complexity parameter

* Use dynamic number of receipts while benchmarking

As part of this change we have removed the hardcoded TEST_RECEIPT_ROOT
and instead chose to calculate the receipt root on the fly. This will
make tests and benches less fragile.

* Prune a dynamic number of headers

bridges/bin/node/runtime/src/lib.rs

@@ -215,6 +215,28 @@ impl pallet_aura::Trait for Runtime {
 	type AuthorityId = AuraId;
 }
 
+// We want to use a different validator configuration for benchmarking than what's used in Kovan,
+// but we can't configure a new validator set on the fly which means we need to wire the runtime
+// together like this
+#[cfg(feature = "runtime-benchmarks")]
+use pallet_bridge_eth_poa::{ValidatorsConfiguration, ValidatorsSource};
+
+#[cfg(feature = "runtime-benchmarks")]
+parameter_types! {
+	pub const FinalityVotesCachingInterval: Option<u64> = Some(16);
+	pub KovanAuraConfiguration: pallet_bridge_eth_poa::AuraConfiguration = kovan::kovan_aura_configuration();
+	pub KovanValidatorsConfiguration: pallet_bridge_eth_poa::ValidatorsConfiguration = bench_validator_config();
+}
+
+#[cfg(feature = "runtime-benchmarks")]
+fn bench_validator_config() -> ValidatorsConfiguration {
+	ValidatorsConfiguration::Multi(vec![
+		(0, ValidatorsSource::List(vec![[1; 20].into()])),
+		(1, ValidatorsSource::Contract([3; 20].into(), vec![[1; 20].into()])),
+	])
+}
+
+#[cfg(not(feature = "runtime-benchmarks"))]
 parameter_types! {
 	pub const FinalityVotesCachingInterval: Option<u64> = Some(16);
 	pub KovanAuraConfiguration: pallet_bridge_eth_poa::AuraConfiguration = kovan::kovan_aura_configuration();