Still needs implementing format_as_block, and switching input and output
to use sys.stdin.readline() and sys.stdout.write(line).

Normal constants, like most other things, are cleaned up during
RSHUTDOWN.
We need to explicitly mark them as persistent using CONST_PERSISTENT

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.