Do not allow a taker to take multiple orders at once

[Catrya]

To prevent a malicious user from "locking" orders by taking them and then not continuing, which would leave the order book empty for a while, a taker should be prevented from having more than 1 order in status waiting-buyer-invoice or waiting-payment.

If they want to take another order, they must complete the one they already took or cancel it if they is not going to continue.