Permit streaming_callback of AsyncHTTPClient to be a coroutine.

[eulersIDcrisis]

This PR adds the ability for streaming_callback of AsyncHTTPClient to be a coroutine. In addition to the small change, this adds a nominal test to verify that things work as expected, at least as best as the streaming_callback approach can permit.

This PR is intended to address a few separate issues listed below:

• Support streaming API for AsyncHTTPClient  #2209
• how to handle the http stream data without using callback but based on coroutine? #3351

[eulersIDcrisis]

@bdarnell , if it is any consolation, I've run tox which passes for me (including the docs and linter checks).