Touch a completed slice when marking a burrow for liquidation

[gkaracha]

As we have discussed at some point in the past, it might be a good idea to make entrypoint_mark_for_liquidation trigger the garbage collection of a slice from a completed auction. In degenerate cases the liquidation queue can still grow faster than is garbage collected of course, but in many cases this extension would help average out the number of slices being sent to auctions.

Of course this change would increase the size of and gas costs for entrypoint_mark_for_liquidation but I think that this might be OK (as of the writing this issue entrypoint_mark_for_liquidation is ~18321 bytes).