Look into dropping rolled back primary during GC

In #655 a change was made to drop rolled back data during GC. This change did not drop delete locks for the primary row/column of a transactions. I suspect it would be safe to drop this under certain circumstances, but not 100% sure. Make this change would require recovery code to equate non-existance with a transaction being rolled back. I thought of this while working on #655, but fixed that in a more conservative way.