BEEFY: define on-chain beefy-genesis in the pallet

[acatangiu]

In order to deploy full beefy (where 1st block in each session is beefy mandatory) we need to define an initial "genesis session" that all validators agree upon.

I initially tried using actual chain genesis for this in https://github.com/paritytech/devops/issues/2198, but (in retrospect obviously) old session validators and keys are no longer available around to retroactively beefy-finalize old blocks.

New plan is to define an on-chain pallet-beefy genesis to mark the beginning of "full beefy consensus". Validators will start running "full beefy" starting with the session where the new pallet is deployed on-chain.