Execute websocket failure on secure tunnel event loop

[sfod]

Host resolution failure could lead to a WebSocket setup callback to be called on a different thread than secure tunnel event loop uses. Use a cross-thread task to handle that particular case.

This PR helps with aws/aws-iot-device-sdk-cpp-v2#669

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

[sbSteveK]

This looks good but I'm gonna hold off on approving because I think we may need to implement a fix in aws-c-http. On the creation of the websocket via aws_websocket_client_connect we pass along options that include requested_event_loop. This should force everything the websocket is doing and/or failing to the provided event loop. The fix here insures we stay on that loop but if it's broken here, it's possible we could be running into similar issues with websocket elsewhere.

Notes:
We originally implemented event loop assignment here: awslabs/aws-c-http#391

[sfod]

Closing in favor of awslabs/aws-c-io#618