BEEFY: add on-chain mechanism to initialize/reset beefy pallet and client (voter)

[acatangiu]

BEEFY will be deployed on running chains so relying on GenesisConfig is not enough.

Currently, to initialize it, sudo/governance needs to do multiple synchronized setStorage** calls (setting pallet genesis block and setting initial authority_set_id to GENESIS_AUTHORITY_SET_ID).

This approach doesn't allow us to set a genesis further in the future than current session because the authority-set-id is incremented every session and the client expects active.id() == GENESIS_AUTHORITY_SET_ID for the pallet genesis block; and is somewhat error-prone.

Solution(s)

1. Relax requirement that initial authority_set_id needs to be GENESIS_AUTHORITY_SET_ID. (easy)
2. Add explicit reset/init mechanism to pallet-beefy so that it only starts tracking and rotating authority-sets after pallet genesis block. (more involved)

CC @andresilva

[acatangiu]

I think we should go with 1st option.

BEEFY consensus can be restarted by resetting "genesisBlock" in pallet-beefy, but we don't want to also reset authority set IDs so that they are uniquely identified across the entire chain history regardless of how many times BEEFY consensus has been reset/restarted.
Ergo the client should also accept initial authority_set_id != 0.