New slashing mechanism (#554)

* Slashing improvements

- unstake when balance too low
- unstake after N slashes according to val prefs
- don't early-terminate session/era unless unstaked
- offline grace period before punishment

* Fix warning

* Cleanups and ensure slash_count decays

* Bump authoring version and introduce needed authoring stub

* Rename

* Fix offline tracker

* Fix offline tracker

* Renames

* Add test

* Tests

* Tests.