No temporary loop for ASGI middleware (#148)

* Yield a disconnect on the second receive call

* Use asyncio.run to handle existing/new event loop for Python 3.7+

---------

Co-authored-by: peterstone2017 <12449837+YunchuWang@users.noreply.github.com>
Co-authored-by: gavin-aguiar <80794152+gavin-aguiar@users.noreply.github.com>

Author: 3 people

azure/functions/_http_asgi.py

@@ -150,7 +150,6 @@ def __init__(self, app):
             self._usage_reported = True
 
         self._app = app
-        self._loop = asyncio.new_event_loop()
         self.main = self._handle
 
     def handle(self, req: HttpRequest, context: Optional[Context] = None):
@@ -172,9 +171,8 @@ async def main(req, context):
 
     def _handle(self, req, context):
         asgi_request = AsgiRequest(req, context)
-        asyncio.set_event_loop(self._loop)
         scope = asgi_request.to_asgi_http_scope()
-        asgi_response = self._loop.run_until_complete(
+        asgi_response = asyncio.run(
             AsgiResponse.from_app(self._app, scope, req.get_body())
         )