Right now users of TaskExecutor that need to work with CallbackHandles have to work around an inherent race between dealing with the returned Handle and the callback which may be running. This race could be avoided if it was possible to create the CallbackHandle in advance then tell the TaskExecutor to use it.

An additional goal to achieve in the process of this work is to, instead of taking a callback, return a future when a CallbackHandle is provided ahead of time.