OAK-1639 - MarkSweepGarbageCollector improvements

-- Fixed log messages
-- Made the blob gc age configurable for DocumentNodeStore
-- Fixed an issue with GC logic which was using maxGCInterval as time

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1584333 13f79535-47bb-0310-9956-ffa450edef68