frame/beefy: prune entries in set id session mapping (#13411)

Add limit for the number of entries in the `SetIdSession` mapping.
For example, it can be set to the bonding duration (in sessions).

Signed-off-by: acatangiu <adrian@parity.io>

substrate/frame/beefy-mmr/src/mock.rs

@@ -133,6 +133,7 @@ impl pallet_beefy::Config for Test {
 	)>>::IdentificationTuple;
 	type HandleEquivocation = ();
 	type MaxAuthorities = ConstU32<100>;
+	type MaxSetIdSessionEntries = ConstU64<100>;
 	type OnNewValidatorSet = BeefyMmr;
 	type WeightInfo = ();
 }