Don't create an internal cursor to execute a command with OP_QUERY.

Still to do:

• Get a new struct, mongoc_server_stream_t, that wraps a stream and server description
• Update cluster to return mongoc_server_stream_t's with info from ismaster call on that stream
• Reduce calls to mongoc_cluster_get_max_bson_obj_size and co., get those values from the mongoc_server_stream_t
• Reduce calls to mongoc_topology_server_by_id, prefer to pass around mongoc_server_stream_t's rather than "hints" or server_id's
• Reduce need to pass is_write_command around.
• Reduce calls to topology_select in places like mongoc_collection_aggregate: get a stream and server description at the same time
• Put $readPreference into the BSON as early as possible, in order to
  simplify run_command.
• Refactor error-checking that's duplicated between my new
  _mongoc_rpc_parse_error and the existing mongoc_cursor_error DONE