Gloas process epoch

[shane-moore]

Changes per spec

• process_slot modified (per_slot_processing)
• process_epoch modified (per_epoch_processing)
  • process_builder_pending_payments added
  • get_builder_payment_quorum_threshold added

How this works

process_slot will now unset the next slot's BeaconState. execution_payload_availability, which contains a ring buffer indexed per slot. We clear the next slot's bit at every slot boundary as to start the slot off as a "no payload yet" state.

process_epoch will now run process_builder_pending_payments at the end. The sole focus of this new function is to loop over the state.builder_pending_payments left over from the previous epoch that did not get converted to state.builder_pending_withdrawals during the fast past conversion because they were associated with CL blocks that did not release their corresponding execution envelopes. So process_builder_pending_payments will check that if the accumulated weights for these state.builder_pending_payments from last epoch are higher than a quorum value. If so, they'll be converted to builder_pending_withdrawal with a withdrawable_epoch.

@ethDreamer