crypto optionality for async module + market hours feed async to be iex as a default

[silvervadim]

Hi I originally had a contribution for the rest_async module to have a crypto option. It passed all of the tests but was delayed approval for a month after Rahul asked to review it (I'm still waiting on that). I closed the earlier pull request and just combined that with this one. Please let me know if you can approve this since I am working on a package I would like to deploy to PyPI