backing: move the min votes threshold to the runtime (#1200)

* move min backing votes const to runtime

also cache it per-session in the backing subsystem

Signed-off-by: alindima <alin@parity.io>

* add runtime migration

* introduce api versioning for min_backing votes

also enable it for rococo/versi for testing

* also add min_backing_votes runtime calls to statement-distribution

this dependency has been recently introduced by async backing

* remove explicit version runtime API call

this is not needed, as the RuntimeAPISubsystem already takes care
of versioning and will return NotSupported if the version is not
right.

* address review comments

- parametrise backing votes runtime API with session index
- remove RuntimeInfo usage in backing subsystem, as runtime API
caches the min backing votes by session index anyway.
- move the logic for adjusting the configured needed backing votes with the size of the backing group
to a primitives helper.
- move the legacy min backing votes value to a primitives helper.
- mark JoinMultiple error as fatal, since the Canceled (non-multiple) counterpart is also fatal.
- make backing subsystem handle fatal errors for new leaves update.
- add HostConfiguration consistency check for zeroed backing votes threshold
- add cumulus accompanying change

* fix cumulus test compilation

* fix tests

* more small fixes

* fix merge

* bump runtime api version for westend and rollback version for rococo

---------

Signed-off-by: alindima <alin@parity.io>
Co-authored-by: Javier Viola <javier@parity.io>

polkadot/runtime/parachains/src/configuration/tests.rs

@@ -317,6 +317,7 @@ fn setting_pending_config_members() {
 			on_demand_fee_variability: Perbill::from_percent(3),
 			on_demand_target_queue_utilization: Perbill::from_percent(25),
 			on_demand_ttl: 5u32,
+			minimum_backing_votes: 5,
 		};
 
 		Configuration::set_validation_upgrade_cooldown(
@@ -467,6 +468,11 @@ fn setting_pending_config_members() {
 		.unwrap();
 		Configuration::set_pvf_voting_ttl(RuntimeOrigin::root(), new_config.pvf_voting_ttl)
 			.unwrap();
+		Configuration::set_minimum_backing_votes(
+			RuntimeOrigin::root(),
+			new_config.minimum_backing_votes,
+		)
+		.unwrap();
 
 		assert_eq!(PendingConfigs::<Test>::get(), vec![(shared::SESSION_DELAY, new_config)],);
 	})