DERBY-3155: Make cleanup logic in MergeResultSet more closely resemble the cleanup logic in InsertResultSet; commit derby-3155-15-aa-replumbMergeResultSetCleanup.diff.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1559218 13f79535-47bb-0310-9956-ffa450edef68