babe: allow skipping over empty epochs (#11727)

* babe: allow skipping epochs in pallet

* babe: detect and skip epochs on client

* babe: cleaner epoch util functions

* babe: add test for runtime handling of skipped epochs

* babe: simpler implementation of client handling of skipped epochs

* babe: test client-side handling of skipped epochs

* babe: add comments on client-side skipped epochs

* babe: remove emptyline

* babe: make it resilient to forks

* babe: typo

* babe: overflow-safe math

* babe: add test for skipping epochs across different forks

* Fix tests

* FMT

Co-authored-by: Bastian Köcher <info@kchr.de>