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>