SolrJ should use multipart forms for Solr Cloud

Once SOLR-5590 is fixed, and the proper content-encoding is specified in SolrJ's HttpSolrServer class, SolrJ should completely support multipart forms once more. When that is done, SolrCloud should also use multipart forms, since otherwise the http GET/POST headers may exceed web server limits. See CONNECTORS-839 for a description of the issue. (Once again, the ManifoldCF project overrode SolrJ classes to make the right thing happen, but we'd like to remove our hack eventually.)