Fixed an issue affecting the thread safeness of the SequenceBank objects.
Added two unit tests to confirm the proper behavior of the SequenceUtil class; the second test fails without the fix committed with this revision.
Part of OFBIZ-2353.
This is just the first pass of my work to refactor the SequenceUtil.


git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1621335 13f79535-47bb-0310-9956-ffa450edef68