OPENJPA-2437 transactional listeners added too late to observe begin event

txs 2 rmannibucau for the fix.
Patch applied with cleanup and tests pimped. 



git-svn-id: https://svn.apache.org/repos/asf/openjpa/branches/2.3.x@1536596 13f79535-47bb-0310-9956-ffa450edef68