SOLR-6592: Avoid waiting for the leader to see the down state if that leader is not live.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1629719 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6511: Better handling of ZooKeeper related exceptions when deciding to start the leader-initiated recovery nag thread

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1629720 13f79535-47bb-0310-9956-ffa450edef68