Validator consolidation via activation churn

[mkalinin]

A simplified version of #12 that does not require changes to the slashing logic.

On process_consolidation in a block:

• Make the source exit with the exit_epoch = compute_activation_exit_epoch(get_current_epoch(state)) bypassing the exit churn
• Queue the consolidation with consolidation_epoch = source.withdrawable_epoch

On epoch_processing:

• If current_epoch == pending_consolidation.epoch and the source wasn’t slashed move the active balance
• The active balance is simply added to the activation churn as a top up of the target validator

Downsides:

• Waiting for withdrawal_epoch to finish consolidation
• Time needed to activate consolidated balance is unpredictable as the activation queue may get longer while keep waiting for the withdrawal_epoch

[mkalinin]

Closing in favour of #14