Makes MergeTableRegionsProcedure do more than just two regions at a
time. Compatible as MTRP was done considering one day it'd do more than
two at a time.
Changes hardcoded assumption that merge parent regions are named
mergeA and mergeB in a column on the resultant region. Instead
can have N columns on the merged region, one for each parent
merged. Column qualifiers all being with 'merge'.
Most of code below is undoing the assumption that there are two
parents on a merge only.
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