Use `TaskCreationOptions.RunContinuationsAsynchronously`

[andyleejordan]

You can kind of think of it like .ConfigureAwait(false) for completion sources. I didn't necessarily need to add it here, it was just a troubleshooting step, but we probably should be adding it everywhere. Related blog post

I can add a comment here but maybe I should just update all the other instances as well? If we go that route would you still prefer a comment over each one?

Originally posted by @SeeminglyScience in #1799 (comment)