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/westend/src/lib.rs

@@ -62,7 +62,9 @@ use runtime_parachains::{
 	inclusion::{AggregateMessageOrigin, UmpQueueId},
 	initializer as parachains_initializer, origin as parachains_origin, paras as parachains_paras,
 	paras_inherent as parachains_paras_inherent, reward_points as parachains_reward_points,
-	runtime_api_impl::v5 as parachains_runtime_api_impl,
+	runtime_api_impl::{
+		v5 as parachains_runtime_api_impl, vstaging as parachains_staging_runtime_api_impl,
+	},
 	scheduler as parachains_scheduler, session_info as parachains_session_info,
 	shared as parachains_shared,
 };
@@ -1422,6 +1424,7 @@ pub mod migrations {
 		parachains_scheduler::migration::v1::MigrateToV1<Runtime>,
 		parachains_configuration::migration::v8::MigrateToV8<Runtime>,
 		UpgradeSessionKeys,
+		parachains_configuration::migration::v9::MigrateToV9<Runtime>,
 	);
 }
 
@@ -1557,6 +1560,7 @@ sp_api::impl_runtime_apis! {
 		}
 	}
 
+	#[api_version(6)]
 	impl primitives::runtime_api::ParachainHost<Block, Hash, BlockNumber> for Runtime {
 		fn validators() -> Vec<ValidatorId> {
 			parachains_runtime_api_impl::validators::<Runtime>()
@@ -1687,6 +1691,10 @@ sp_api::impl_runtime_apis! {
 				key_ownership_proof,
 			)
 		}
+
+		fn minimum_backing_votes() -> u32 {
+			parachains_staging_runtime_api_impl::minimum_backing_votes::<Runtime>()
+		}
 	}
 
 	impl beefy_primitives::BeefyApi<Block, BeefyId> for Runtime {