Rework priority groups, take 2 (#7700)

* Rework priority groups

* Broken tests fix

* Fix warning causing CI to fail

* [Hack] Try restore backwards-compatibility

* Fix peerset bug

* Doc fixes and clean up

* Error on state mismatch

* Try debug CI

* CI debugging

* [CI debug] Can I please see this line

* Revert "[CI debug] Can I please see this line"

This reverts commit 4b7cf7c1511f579cd818b21d46bd11642dfac5cb.

* Revert "CI debugging"

This reverts commit 9011f1f564b860386dc7dd6ffa9fc34ea7107623.

* Fix error! which isn't actually an error

* Fix Ok() returned when actually Err()

* Tweaks and fixes

* Fix build

* Peerset bugfix

* [Debug] Try outbound GrandPa slots

* Another bugfix

* Revert "[Debug] Try outbound GrandPa slots"

This reverts commit d175b9208c088faad77d9f0ce36ff6f48bd92dd3.

* [Debug] Try outbound GrandPa slots

* Apply suggestions from code review

Co-authored-by: Max Inden <mail@max-inden.de>

* Use consts for hardcoded peersets

* Revert "Try debug CI"

This reverts commit 62c4ad5e79c03d561c714a008022ecac463a597e.

* Renames

* Line widths

* Add doc

Co-authored-by: Max Inden <mail@max-inden.de>