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. 
I think we could. If we're going to commit to that, there's a much bigger commit to `s/[sS]olr_?//` across the repository.