OAK-631: SegmentMK: Implement garbage collection

The return value of collectFiles is no longer a SortedMap,
use TreeSet to explicitly order the returned list of index numbers

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1584381 13f79535-47bb-0310-9956-ffa450edef68