This rework fixes a bug in the pre-code-gen time code. The bug was
that in the presence of partitioning key predicates, logic that
communicates stop column information to the generator was being
bypassed. This would result in traversal to the last key predicate
at run time, with poor performance if the stop column was prior to
the last key predicate.

Also included in this rework are some tweaks to the run-time debug
code for MDAM (the code had become non-functional from not being used
for a long time), and a fix to a run-time memory leak bug that shows
up as an assert when that debug code is present.