[pallet_broker] Remove leases that have already expired in rotate_sale (#3213)

Leases can be force set, but since `Leases` is a `StorageValue`, if a
lease misses its sale rotation in which it should expire, it can never
be cleared.

This can happen if a lease is added with an `until` timeslice that lies
in a region whose sale has already started or has passed, even if the
timeslice itself hasn't passed.

This solves that issue in a minimal way, with all expired leases being
cleaned up in each sale rotation, not just the ones that are expiring in
the coming region.

TODO:
- [x] Write test

substrate/frame/broker/src/tests.rs

@@ -863,6 +863,29 @@ fn cannot_set_expired_lease() {
 	});
 }
 
+#[test]
+fn short_leases_are_cleaned() {
+	TestExt::new().region_length(3).execute_with(|| {
+		assert_ok!(Broker::do_start_sales(200, 1));
+		advance_to(2);
+
+		// New leases are allowed to expire within this region given expiry > `current_timeslice`.
+		assert_noop!(
+			Broker::do_set_lease(1000, Broker::current_timeslice()),
+			Error::<Test>::AlreadyExpired
+		);
+		assert_eq!(Leases::<Test>::get().len(), 0);
+		assert_ok!(Broker::do_set_lease(1000, Broker::current_timeslice().saturating_add(1)));
+		assert_eq!(Leases::<Test>::get().len(), 1);
+
+		// But are cleaned up in the next rotate_sale.
+		let config = Configuration::<Test>::get().unwrap();
+		let timeslice_period: u64 = <Test as Config>::TimeslicePeriod::get();
+		advance_to(timeslice_period.saturating_mul(config.region_length.into()));
+		assert_eq!(Leases::<Test>::get().len(), 0);
+	});
+}
+
 #[test]
 fn leases_are_limited() {
 	TestExt::new().execute_with(|| {