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

Support for gc of deleted nodes

-- Prefer reading from secondary
-- Fix MongoFixture to create a new DocumentStore for every call

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582349 13f79535-47bb-0310-9956-ffa450edef68