Completely refactored the SequenceUtil class:
* the class is now thread safe
* the class is now cleaner (no references to delegator etc...)
* replaced the collision detection algorithm with a simpler/reliable/efficient one based on: update with no changes (to get a lock) + select + update (to increase the last sequence id)
* removed ugly special code for "clusters" that is no more needed



git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1621438 13f79535-47bb-0310-9956-ffa450edef68