Override RuntimeApi::authorities() to include past sessions (#2494)

* override authorities runtime call

* addressing feedback

* addressing feedback and restoring semantics

* updated helper function

* naming convention + comment

Co-authored-by: Parity <parity2305@gmail.com>

polkadot/runtime/rococo/src/lib.rs

@@ -849,7 +849,7 @@ sp_api::impl_runtime_apis! {
 
 	impl authority_discovery_primitives::AuthorityDiscoveryApi<Block> for Runtime {
 		fn authorities() -> Vec<AuthorityDiscoveryId> {
-			AuthorityDiscovery::authorities()
+			runtime_api_impl::relevant_authority_ids::<Runtime>()
 		}
 	}