Support "stats" queries [JIRA: RIAK-2711]

[kesslerm]

Solr supports "stats" queries. When the appropriate query parameters are passed through the Yokozuna endpoint, they appear to be passed on to the local Solr core, which returns a result. However, as coverage queries are not taken into account, the results may fluctuate based on which cluster node actually coordinates the Solr query.

Yokozuna should be enhanced to use an appropriate coverage query plan for "stats".

[kesslerm]

The groundwork for supporting additional query parameters on top of coverage plans may already have been laid as part of ongoing work on #309.