SOLR-6136: Fix spin lock in blockUntilFinished (which eats CPU unnecessarily) using wait/notify and add a unit test for ConcurrentUpdateSolrServer

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1610864 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6179: Better strategy for handling empty managed data to avoid spurious warning messages in the logs.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1610870 13f79535-47bb-0310-9956-ffa450edef68