Do not store withdrawal_credentials

[JustinDrake]

The Validator object does not need to store withdrawal_credentials. Instead, we can have a stateless approach where withdrawal operations include a Merkle proof to a historical deposit via historical_roots. To make this safe we can check that deposit.data.withdrawal_credentials == ZERO_HASH for deposits to existing validators.

The general heuristic is that consensus storage is much more expensive than consensus bandwidth. This is especially true for withdrawal_credentials which is read exactly once. In the worst case we can save over 100MB from the beacon state.

[JustinDrake]

Closing this issue because withdrawal_credentialsdoes not need to be stored in RAM by validators (it can be stored on disk). It may also be possible to implement statelessness with without consensus support.

@ralexstokes: You gave a thumbs up. Feel free to reopen the issue :)