OAK-1341 - DocumentNodeStore: Implement revision garbage collection (WIP)

Support for gc of deleted nodes

-- Mark docs once deleted with a flag
-- Created an index on _deletedOnce flag
-- Updated test fixture for easier use

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582348 13f79535-47bb-0310-9956-ffa450edef68