As of CDRIVER-3905 mongoc_collection_estimated_document_count can return a different server response format depending on whether the command was run against < 4.9 or 4.9+.

Create a consistent format for the reply of estimatedDocumentCount. Additionally, the spec only requires us to return an int, so we should investigate the usage of this returned response.

Also, investigate the error that could be returned with a successful but malformed reply in estimated document count.