Don't require the 'text' field

The fact that the `text` field was being set as `df` by the select handler was just artifact of copying Solr example and not paying close enough attention. This removes that mapping so the user must either explicitly use a field for every term or pass the `df` request parameter.

* Use `_yz_id` for the ping handler.
* Remove `df` default from select handler.
* Update basho bench driver.
* Update migration test.

This is related to #306.