Now that voter is initialized from persistent state, it makes
sense that it can attempt voting right away. This also helps
the genesis case when we consider block `One` as mandatory.

Persist state after handling self vote to avoid double voting in case
of voter restarts.

For mandatory blocks we want to make sure we're not losing votes
in case of crashes or restarts, since voter will not make further
progress without finalizing them.

To guarantee unbroken chain of mandatory blocks justifications, voter
will always resume from either last BEEFY-justified block or
`pallet-beefy` genesis, whichever is more recent.

Initialization walks back the chain from latest GRANDPA finalized
block looking for one of the above. Along the way, it also records
and enqueues for processing any BEEFY mandatory blocks that have
been already GRANDPA finalized but not BEEFY finalized.

…ersist-state