Determine validator_set_id at BeefyWorker startup

[adoerr]

BeefyWorker needs to be initialized with the current valid validator_set_id. Right now this id is hard coded to the genesis validator set id '0' (zero).

[adoerr]

I think this should be an Option and we should skip voting if it's not set. Otherwise I'm afraid that BEEFY validators will start signing bullshit commitments. Later on that should obviously be penalized, but currently I think it's safer to just wait for the beginning of the session. Not sure how this will work initially though (i.e. if we ever produce the first commitment) - perhaps special casing empty chain will be required (setting Some(0) instead of None).

Comment from @tomusdrw on #94.

Currently we always use Some(0) as the validator_set_id, otherwise the test deployment would never vote on anything. Reason is that the test deployment always starts with genesis, so we need to implement the empty chain test at the very least for the test deployments.