This defect was introduced in 9e4d0610. I had thought that
this code was exercised in tests but it is bypassed by the use of
grpc_test.beta.test_utilities.not_really_secure_channel. :-(

A transport op indicating a cancellation can be sent to the auth client
filter. In this case, the code should not assert that a context is not
null on this op.

Fixes #3203

We entered a live lock race between the promotion callback and pollers,
which was only resolved when the background executor got lucky enough to
pick up the async work.

Failing to use await can result in deadlock. This is likely to be the cause of shutdown hangs in #3347.

These changes probably should have been in 3b0fefb2 but
were overlooked.

The old package directory handling was stale in grpcio's setup.py
command support module and docgen had a typo.

- We cannot use call_data after the final callback as this one may
  trigger the deletion of the call object (hence the call_data).

I *believe* this is actually safe, and the assert was errantly
copy-pasted a while back.

Fixes #3022