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

- move blob store base implementations to oak.spi.blob package
- tmp move to separate oak-blob bundle in order to avoid circular dependency between oak-mk and oak-core


git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1570281 13f79535-47bb-0310-9956-ffa450edef68