SOLR-4260: ConcurrentUpdateSolrServer#blockUntilFinished can return before all previously added updates have finished. This could cause distributed updates meant for replicas to be lost.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1558979 13f79535-47bb-0310-9956-ffa450edef68