OAK-1708 - extend DocumentNodeStoreService to support RDBPersistence

Added support for referencing the DataSource via OSGi service and also added relevant testcase in pojosr module

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1599472 13f79535-47bb-0310-9956-ffa450edef68