Modify producer api to increase flexibility

**FEATURE REQUEST**

```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.