SOLR-6860: Re-enable REBALANCELEADERS for 5.1. going to let this bake in trunk only until 5.0 is cut

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1647849 13f79535-47bb-0310-9956-ffa450edef68
SOLR=6691: REBALANCELEADERS needs to change the leader election queue. Going to let this bake in trunk until 5.0 is cut

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1647857 13f79535-47bb-0310-9956-ffa450edef68