construct mmr leaf prior to session pallet hook (#6577)

* move pallet_mmr ahead of pallet_session

addresses #11797

* document construction of mmr leaf prior to session

runtime/rococo/src/lib.rs

@@ -1352,6 +1352,9 @@ construct_runtime! {
 		Authorship: pallet_authorship::{Pallet, Call, Storage} = 5,
 		Offences: pallet_offences::{Pallet, Storage, Event} = 7,
 		Historical: session_historical::{Pallet} = 34,
+		// MMR leaf construction must be before session in order to have leaf contents
+		// refer to block<N-1> consistently. see substrate issue #11797 for details.
+		Mmr: pallet_mmr::{Pallet, Storage} = 241,
 		Session: pallet_session::{Pallet, Call, Storage, Event, Config<T>} = 8,
 		Grandpa: pallet_grandpa::{Pallet, Call, Storage, Config, Event, ValidateUnsigned} = 10,
 		ImOnline: pallet_im_online::{Pallet, Call, Storage, Event<T>, ValidateUnsigned, Config<T>} = 11,
@@ -1436,7 +1439,6 @@ construct_runtime! {
 		//
 		// BEEFY Bridges support.
 		Beefy: pallet_beefy::{Pallet, Storage, Config<T>} = 240,
-		Mmr: pallet_mmr::{Pallet, Storage} = 241,
 		MmrLeaf: pallet_beefy_mmr::{Pallet, Storage} = 242,
 
 		ParasSudoWrapper: paras_sudo_wrapper::{Pallet, Call} = 250,