SOLR-5476 avoiding race condition 

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1566247 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5624: Guard against NPE during cache warming

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1566309 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5710: Migrate collection command does not handle not finding a shard leader well

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1566385 13f79535-47bb-0310-9956-ffa450edef68