CDRIVER-2331 made a mess of our write command path, especially how it handles "opts". Some "opts" documents apply to individual operations within a write command, e.g. opts that contain "arrayFilters", other "opts" apply to the entire write command, e.g. opts that contain "writeConcern". Some options are contained in BSON documents, others are in the (unfortunately public and immutable) mongoc_bulk_write_flags_t struct.