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/network/statement-distribution/Cargo.toml

@@ -16,6 +16,7 @@ sp-keystore = { path = "../../../../substrate/primitives/keystore" }
 polkadot-node-subsystem = { path = "../../subsystem" }
 polkadot-node-primitives = { path = "../../primitives" }
 polkadot-node-subsystem-util = { path = "../../subsystem-util" }
+polkadot-node-subsystem-types = { path = "../../subsystem-types" }
 polkadot-node-network-protocol = { path = "../protocol" }
 arrayvec = "0.7.4"
 indexmap = "1.9.1"
@@ -39,4 +40,3 @@ sc-network = { path = "../../../../substrate/client/network" }
 futures-timer = "3.0.2"
 polkadot-primitives-test-helpers = { path = "../../../primitives/test-helpers" }
 rand_chacha = "0.3"
-polkadot-node-subsystem-types = { path = "../../subsystem-types" }