liquidation 2.0: add bark's tip to tab (user debt)

[yaronvel]

In liquidation 1.2, bite cost over $50, and during network congestion this could be tripled. Currently the tip is paid (to my understanding) by the system. This will force high dust, where the liquidation penalty exceeds the proposed tip.

By making the tip part of the liquidation penalty, lower dust levels could be supported.