Respect minimum active+inactive stake when unstaking

[sisou]

We are observing a high number of retire-stake transactions that fail to execute because of a coinvalue error.

Together with @paberr we think that that happens because users have deactivated their stake, waited until the end of the next epoch and then clicked the enabled unstake button. However, the reason these txs fail is that in the meantime, their pool has issued a restaking payout, adding a small balance to their active stake again.

If that added active stake is less than the minimum stake (which it usually is), the retire tx fails because the combined active+inactive stake must be 0 or >= MIN_STAKE, which it would not be after the tx.

To not block users from unstaking though, we should automatically adjust the unstaked balance to let MIN_STAKE remain on the staker (as combined active and inactive balance) and let them unstake the rest, which is usually much more than that minimum.