Optimize CancelationToken-usage in TaskExecutor to avoid extra call to schedule

The first implementation of SERVER-50658 leads to running callbacks on network responses being less efficient, since there are two calls to schedule the callback in the thread pool rather than one. (Once to trigger the output future, and another to run the callback.) This ticket is to optimize it to be as efficient as it was before.

SERVER-52916 also has the same issue compared to scheduleWorkAt. This ticket is to optimize all ExecutorFuture-returning functions in TaskExecutor.