OAK-2019 Compact only if needed
 - exposed pause flag via OSGi config

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1619808 13f79535-47bb-0310-9956-ffa450edef68
OAK-2039 SegmentNodeStore might not create a checkpoint
 - added warning logs when checkpoint creation fails

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1619815 13f79535-47bb-0310-9956-ffa450edef68