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/primitives/src/runtime_api.rs

@@ -240,6 +240,13 @@ sp_api::decl_runtime_apis! {
 			key_ownership_proof: vstaging::slashing::OpaqueKeyOwnershipProof,
 		) -> Option<()>;
 
+		/***** Staging *****/
+
+		/// Get the minimum number of backing votes for a parachain candidate.
+		/// This is a staging method! Do not use on production runtimes!
+		#[api_version(6)]
+		fn minimum_backing_votes() -> u32;
+
 		/***** Asynchronous backing *****/
 
 		/// Returns the state of parachain backing for a given para.