SOLR-4616: Make HitRatio into a float in mbeans

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1478450 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-4973: Persistent/SnapshotDeletionPolicy no longer require a unique id

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1478452 13f79535-47bb-0310-9956-ffa450edef68