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/node/core/runtime-api/src/lib.rs

@@ -101,6 +101,9 @@ where
 				self.requests_cache.cache_authorities(relay_parent, authorities),
 			Validators(relay_parent, validators) =>
 				self.requests_cache.cache_validators(relay_parent, validators),
+			MinimumBackingVotes(_, session_index, minimum_backing_votes) => self
+				.requests_cache
+				.cache_minimum_backing_votes(session_index, minimum_backing_votes),
 			ValidatorGroups(relay_parent, groups) =>
 				self.requests_cache.cache_validator_groups(relay_parent, groups),
 			AvailabilityCores(relay_parent, cores) =>
@@ -301,6 +304,15 @@ where
 			Request::StagingAsyncBackingParams(sender) =>
 				query!(staging_async_backing_params(), sender)
 					.map(|sender| Request::StagingAsyncBackingParams(sender)),
+			Request::MinimumBackingVotes(index, sender) => {
+				if let Some(value) = self.requests_cache.minimum_backing_votes(index) {
+					self.metrics.on_cached_request();
+					let _ = sender.send(Ok(value));
+					None
+				} else {
+					Some(Request::MinimumBackingVotes(index, sender))
+				}
+			},
 		}
 	}
 
@@ -551,6 +563,12 @@ where
 			ver = Request::SUBMIT_REPORT_DISPUTE_LOST_RUNTIME_REQUIREMENT,
 			sender
 		),
+		Request::MinimumBackingVotes(index, sender) => query!(
+			MinimumBackingVotes,
+			minimum_backing_votes(index),
+			ver = Request::MINIMUM_BACKING_VOTES_RUNTIME_REQUIREMENT,
+			sender
+		),
 
 		Request::StagingParaBackingState(para, sender) => {
 			query!(