SOLR-4909: Use DirectoryReader.openIfChanged in non-NRT mode

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1521563 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5210: make perSegFilter cache actually per-segment

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1521564 13f79535-47bb-0310-9956-ffa450edef68