LUCENE-5263: if merge hits exc when releasing merged reader then drop changes and remove it from the pool, since the merge will be aborted

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1531836 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5263: fix comment

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1531837 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5338: Split shards by a route key using split.key parameter

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1531845 13f79535-47bb-0310-9956-ffa450edef68