[ISSUE #916] Modify producer api to increase flexibility

[horoc]

What is the purpose of the change

916

RequestAsync has an argument callback which is an internal function type.

Maybes it's better to define an API function without importing an internal definition.

For example:

1. When I tried to implement a mock producer to do unit testing of my business logic, it was almost impossible.
2. When I tried to implement a wrapper producer which integrated some custom logic, it was also very hard to do.

Brief changelog

change callback type to raw function definition.