LUCENE-5843: IndexWriter now enforces max docs in one index

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1614402 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6020: Auto-generate a unique key in schema-less example if data does not have an id field

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1614417 13f79535-47bb-0310-9956-ffa450edef68