SOLR-5795: New DocExpirationUpdateProcessorFactory supports computing an expiration date for documents from the TTL expression, as well as automatically deleting expired documents on a periodic basis (merge r1583734)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1583741 13f79535-47bb-0310-9956-ffa450edef68