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/subsystem-util/src/runtime/error.rs

@@ -33,7 +33,7 @@ pub enum Error {
 	/// Some request to the runtime failed.
 	/// For example if we prune a block we're requesting info about.
 	#[error("Runtime API error {0}")]
-	RuntimeRequest(RuntimeApiError),
+	RuntimeRequest(#[from] RuntimeApiError),
 
 	/// We tried fetching a session info which was not available.
 	#[error("There was no session with the given index {0}")]
@@ -43,7 +43,7 @@ pub enum Error {
 pub type Result<T> = std::result::Result<T, Error>;
 
 /// Receive a response from a runtime request and convert errors.
-pub(crate) async fn recv_runtime<V>(
+pub async fn recv_runtime<V>(
 	r: oneshot::Receiver<std::result::Result<V, RuntimeApiError>>,
 ) -> Result<V> {
 	let result = r