LUCENE-5461: fix thread hazard in ControlledRealTimeReopenThread causing a possible too-long wait time when a thread was waiting for a specific generation

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_7@1570562 13f79535-47bb-0310-9956-ffa450edef68