Grandpa Pallet Pruning (#890)

* Pruning.

* Add tests.

* Address review.

bridges/modules/grandpa/src/mock.rs

@@ -81,6 +81,7 @@ impl frame_system::Config for TestRuntime {
 
 parameter_types! {
 	pub const MaxRequests: u32 = 2;
+	pub const HeadersToKeep: u32 = 5;
 	pub const SessionLength: u64 = 5;
 	pub const NumValidators: u32 = 5;
 }
@@ -88,6 +89,7 @@ parameter_types! {
 impl grandpa::Config for TestRuntime {
 	type BridgedChain = TestBridgedChain;
 	type MaxRequests = MaxRequests;
+	type HeadersToKeep = HeadersToKeep;
 	type WeightInfo = ();
 }