Payout transaction type

[jasny]

Add a new transaction type: Payout.

Payout transactions are similar to mass-transfer txs. However, the amount is automatically leased to the sender.

Downside is that a holder will have many small leases that would need to be canceled individually. For this to work smoothly we might also need to change the logic for canceling a lease. Instead of a tx id, you'd specify a recipient and amount to unlease.