Backports and a version bump to v0.5.1 (#400)

* Allow parallel parachain validation (#370)

* Allow parallel parachain validation

* Fixed test interference

* Switch to pooled implementation

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* Update validation_host.rs

* Minor cleanups

* Fixed build

* Fix up bonding period. (#396)

* Bump runtime version

* Global version bump to 0.5.1

* Bump another couple of versions to 0.5.1

* Disclaimer