Extract SolrRepository class to handle communication with Solr

Extract `Blacklight::SolrRepository` out of `Blacklight::SolrHelper` for handling interaction with Solr. Also, extract the query building parts of `SolrHelper` into `Blacklight::RequestBuilders` so all the solr parameter generation happens in the same place. 
If we're trying to move to a more generic abstraction, how about just "response_model"?