Re-introduce #539, fix abort to not abort non-execute requests

[glentakahashi]

Fixes #571

As described in the original issue (#539), the old stop_on_error_timeout functionality didn't actually work asynchronously, and would cause new requests to block until the error was finished. The PR #539 fixed this to work async without changing any of the other logic.

This caused occasional issues in other libraries that depend on the kernel_info request, when the notebook had cells with python errors in them due to the race condition of sending off the request.

This new PR re-introduces the async stop_on_error_timeout fix, but changes the logic to abort /only/ execute_request's. This means that running multiple cells in a row where the first cell has an error will still cause the remaining cell executions to abort, but you can still continue to do things like kernel_info, inspect, completions, etc.

It also adds a couple of tests for those.

Also tested against papermill in nteract/papermill#519 to verify that the fix allows papermill to still function.

[glentakahashi]

Hmm, i'm pretty sure the failing test is a flake, but I don't know how to re-trigger a step in github actions. Do I just have to push up an empty no-op commit or do I lack permissions?

[blink1073]

Thanks for adding the test and for testing against papermill! I kicked CI. Closing and re-opening the PR works I think, otherwise you can rebase and force push to force a rebuild.

[blink1073]

Thanks! A different CI build failed this time, so it looks like flakiness.