Hardened Eth 1.0 voting strategy

[JustinDrake]

Addresses #722.

(Todo: find a clean way to tie-break the max by distance.)

[djrtwo]

Is there anything left on this @JustinDrake ?

[JustinDrake]

The only thing left is the tie breaker for return max(valid_votes, key=valid_votes.count, default=get_eth1_data(ETH1_FOLLOW_DISTANCE)). Favouring the vote with the smallest distance (as previously specified) makes sense to me.

[JustinDrake]

@protolambda Do you feel like adding a test for get_eth1_vote?

[protolambda]

@JustinDrake We don't quite have an executable validator spec, but I'm testing the 50% and 51% cases for eth1 voting in in the beacon-spec, in addition to a full block-processing based eth1 voting test.