add batching to SolrJsonWriter

I am not too interested in MRI performance compared to JRuby -- because of the GIL, you are never going to get JRuby-levels of total throughput on MRI. 

I am instead interested in if we get get SolrJsonWriter to be as performant as SolrJWriter on JRuby -- then it could be the default on both MRI and JRuby, and we could even get rid of the solrj.jar dependency which would be nice. 

So I added batching to SolrJsonWriter. I used stdlib ruby Queue so it would run on MRI too. 

It is hard to tell how much it helped, because I get SUCH varying results when running on against a real Solr. I think maybe it improved things, but still not to SolrJ levels.  But I'm not entirely sure it improved performance at all. 

@billdueber , if you wanted to do any, even messy, perf comparison of SolrJWriter vs SolrJsonWriter on Jruby, it would be helpful to see if you get what i was getting.