Mv level work3 ... change from 3 overlapped levels to 2

Details here:  https://github.com/basho/leveldb/wiki/Mv-level-work3

Three overlapped levels gives us better performance, especially in write heavy and in read recent heavy environments.  However, the conversion time to shift 1.3 level files into 1.4 formation would have taken hours on big installations.  So only doing one extra overlapped this release.  It will also smooth the way for sorter upgrade from 2 to 3 in a future release.