Fixed #30451 -- Added ASGI handler and coroutine-safety.

This adds an ASGI handler, asgi.py file for the default project layout,
a few async utilities and adds async-safety to many parts of Django.

tests/async/models.py

@@ -0,0 +1,5 @@
+from django.db import models
+
+
+class SimpleModel(models.Model):
+    field = models.IntegerField()