OAK-1434 : Cleanup dependencies of oak-core (WIP)

- minor cleanup in oak-core's kernel test package
- move json package to commons
- move common utilities to commons


git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1570228 13f79535-47bb-0310-9956-ffa450edef68