Update proposal slashing protection to new spec changes

[0xKiwi]

As of ethereum/consensus-specs#1612 , proposal slashings are now on a per slot basis, rather than a per epoch basis. As a result, the proposal slashing protection on the validator client needs to be modified to be aware of this. Currently if a validator proposes once in an epoch, they will be blocked from proposing in that epoch again when it won't be a slashable offense.

[terencechain]

Closed via #5292