sc-consensus-beefy: fix initialization when BEEFY genesis state unavailable (#14752)

When BEEFY voter is initialized from scratch (no aux db persistent data present),
it needs to find BEEFY genesis block and all subsequent Mandatory blocks and
sync justifications for them.

The initialization code was getting active validator sets for these older blocks
from state, but in cases such as 'fast sync', state is unavailable.

This commit adds a fallback initialization mechanism when state is unavailable:
parse header Digests looking for validator set change log deposits.

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