Optimize Model filtering in Solr find_references queries

[escowles]

Valkyrie::Persistence::Solr::QueryService has two methods that do a query and then filters by model:

• find_references_by
• find_inverse_references_by

These should be optimized by passing the model to the query and letting Solr do the filtering, like we do in find_all_of_model

Related to https://github.com/samvera/valkyrie/pull/790