DERBY-6227: Distinct aggregates don't work well with territory-based collation

Make the duplicate elimination use the DataValueDescriptors directly instead
of converting the values to String objects first. This ensures that the
collation rules of the database are used to compare the values, and those
rules may be different from those used by String.equals().

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1603793 13f79535-47bb-0310-9956-ffa450edef68