OPENJPA-2423: Isolation level is not working properly on DB2 for JPQL queries with nested sub-queries - committed to 2.1.x the patch/fix provided by Pinaki Poddar.

git-svn-id: https://svn.apache.org/repos/asf/openjpa/branches/2.1.x@1517838 13f79535-47bb-0310-9956-ffa450edef68