SOLR-6125: Allow SolrIndexWriter to close without waiting for merges

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_9@1603604 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6064: Return DebugComponent track output as JSON object

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_9@1603611 13f79535-47bb-0310-9956-ffa450edef68