More refactoring in get_events_as_list (#5707)

We can now use `_get_events_from_cache_or_db` rather than going right back to
the database, which means that (a) we can benefit from caching, and (b) it
opens the way forward to more extensive checks on the original event.

We now always require the original event to exist before we will serve up a
redaction.

tests/unittest.py

@@ -447,6 +447,7 @@ class HomeserverTestCase(TestCase):
         # Create the user
         request, channel = self.make_request("GET", "/_matrix/client/r0/admin/register")
         self.render(request)
+        self.assertEqual(channel.code, 200)
         nonce = channel.json_body["nonce"]
 
         want_mac = hmac.new(key=b"shared", digestmod=hashlib.sha1)