Stick the whole DXR index in a single shard.

This should be faster, according to the benchmarks at http://blog.trifork.com/2014/01/07/elasticsearch-how-many-shards/.

Also, crank up the refresh interval. This will flush the indexing buffer out to disk less often, resulting in fewer Lucene segments, which should yield faster queries before they merge and less merging to do, meaning less IO and CPU.