Increase length of historical vectors

[JustinDrake]

Address item 11 of #1054. This stores 8-month old historical start shards so that historical committees to be recomputed (e.g. for attester slashings).

Notice this makes slashing quite harsh:

state.validator_registry[slashed_index].withdrawable_epoch = current_epoch + EPOCHS_PER_HISTORICAL_VECTOR

[djrtwo]

Two options to reduce slashing severity

• Just use a different vector length for the slashing accumulation array
• Do not use the entire array of history for calculating the total slashed in the period. Can make total_at_start be something like the following total_at_start = state.latest_slashed_balances[(current_epoch + EPOCHS_PER_HISTORICAL_VECTOR - EPOCHS_PER_SLASHING_PERIOD) % EPOCHS_PER_HISTORICAL_VECTOR]

[vbuterin]

Agree that we don't need to make slashing lead to an 8 month delay; probably best to keep the current ~36 days.

[djrtwo]

@JustinDrake What route do you want to go here?

[JustinDrake]

What route do you want to go here?

"Just use a different vector length for the slashing accumulation array" sounds fine :) (I'm at a zkSNARKs workshop this week—do you mind going ahead on this one?)

[djrtwo]

on it

[djrtwo]

closing in favor of #1196