pallet-beefy: ensure mandatory block once per session (#11269)

* pallet-beefy: ensure mandatory block once per session

Signed-off-by: acatangiu <adrian@parity.io>

* pallet-beefy: fix tests with auth changes every session

Signed-off-by: acatangiu <adrian@parity.io>

* Apply suggestions from code review

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* beefy: fix incorrect skip session metric on node restart

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>