Double Terminate Crash

[bretambrose]

Ensure that the continuation completion callbacks and its decref only get performed once no matter what the situation

Greengrass has a high-level repro and the logs show the completion callback and the decref inside occuring twice for the same continuation, once due to terminate stream flags on the outbound client socket write callback and once due to terminate stream flags received in the meantime from the server's response.

I was not able to come up with a test with the existing framework that triggered this condition, however.

Solution uses an atomic because it's not clear we can guarantee that the completion callback always gets invoked on the event loop thread.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bretambrose]

Per Mike's feedback on the non-atomic version, the non-atomic version doesn't prevent queued completions (via a socket write task) from calling complete after the connection's continuation table has been "cleared."