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

GC of split document
-- Mark split docs depending on types
-- Added new prop and created sparse index on _sdType

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582659 13f79535-47bb-0310-9956-ffa450edef68