Added

• Add support for multiple sanic workers (configurable with the
  ACTION_SERVER_SANIC_WORKERS environment variable).
• Add support for async run methods in the Action class.
• Return status code 404 in case requested action was not found.
• Return status code 400 in case an empty request body was sent to the /webhook
  endpoint.

Changed

• Replace flask server framework with sanic.
• Replace flask_cors with sanic-cors.
• CollectingDispatcher.utter_message can now do anything that other dispatcher
  methods can do.
• The CollectingDispatcher methods utter_custom_message, utter_elements,
  utter_button_message, utter_attachment, utter_button_template,
  utter_template, utter_custom_json and utter_image_url were deprecated in
  favor of utter_message.
• Updated format strings to f-strings where appropriate.

Removed

• Remove requests dependency
• Remove gevent dependency