SolrJ 操作HttpSolrServer, ConcurrentUpdateSolrServer和CloudSolrServer
HttpSolrServer使用了ApacheCommonsHTTP客户端来连接Solr.注意在Solr4.x中, CommonsHttpSolrServer已经改变为HttpSolrServer以及StreamingUpdateSolrServer已经改变为ConcurrentUpdateSolrServer 。ConcurrentUpdateSolrServer更适合update操作,