Moare fixes for parachains (#1911)

* Moare fixes for parachains

- Sending data to a job should always contain a relay parent. Done this
for the provisioner
- Fixed the `select_availability_bitfields` function. It was assuming we
have one core per validator, while we only have one core per parachain.
- Drive by async "rewrite" in proposer

* Make tests compile

* Update primitives/src/v1.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

polkadot/primitives/src/v1.rs

@@ -623,6 +623,11 @@ impl<N> CoreState<N> {
 			Self::Free => None,
 		}
 	}
+
+	/// Is this core state `Self::Occupied`?
+	pub fn is_occupied(&self) -> bool {
+		matches!(self, Self::Occupied(_))
+	}
 }
 
 /// An assumption being made about the state of an occupied core.