OAK-1858 Segment Explorer
 - first draft



git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1597789 13f79535-47bb-0310-9956-ffa450edef68
OAK-1804 TarMK compaction
 - parts relevant to the oak-core

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1597795 13f79535-47bb-0310-9956-ffa450edef68