[P0] `devshards`: Distribute `WorkCoins` at the end of the epoch

[dcastro]

As described in #914 (comment), we want to:

• Distribute WorkCoins at the end of the epoch, instead of upon settlement.
• Take devshards stats into account when
  • calculating punishments WorkCoins/RewardCoins (see bitcoin_rewards.go)
  • participant's inactivity status (see status.go -> ComputeStatus)