Implement index-agnostic Response

[igorcoding]

asynctnt.Response's entries should be agnostic to indexing (either by an index or by a key).
Almost everything is there for implementing it (fields are already provided for each tuple in response), so it should be fairly easy to implement lazy build of dictionaries. body property should be deprecated.
Response should in some way contain tuples that can be accessed either by a key or a numeric index.

This produced some questions with non-space requests (such as call and eval), because there are no actual "tuples" (in sense of space) there.

By implementing this feature parameter tuple_as_dict to all requests would be also declared as deprecated and removed in the future.

Probably should wait until SQL protocol in Tarantool is documented to implement more general solution.