Support async request handlers

[jab]

Flask v2 (due to come out in the next couple weeks) adds limited support for async request handlers, see e.g.:

• http://flask.palletsprojects.com/en/master/async-await/
• https://palletsprojects.com/blog/end-python2-prereleases/

As the docs above say:

Existing Flask extensions only expect views to be synchronous. If they provide decorators to add functionality to views, those will probably not work with async views because they will not await the function or be awaitable. Other functions they provide will not be awaitable either and will probably be blocking if called within an async view.

This issue tracks adding support to Rebar specifically for Flask v2 async request handlers.