Investigate if we can stop ServiceRequestContextBuilder.build() from scheduling an event loop task

[trustin]

DefaultCancellationScheduler.initAndStart() and (CancellationScheduler.finished()) is called by {Service,Client}RequestContextBuilder.build() here. initAndStart() and finished() schedule some of its work to run in an event loop.

This behavior can cause the task queue of the EventLoop to grow unexpectedly big and even trigger OutOfMemoryError under load.

We could prevent this by making the following changes:

• Provide a way to disable cancellation scheduler completely so that no task is scheduled; and
• Consider making it the default behavior.

[Bue-von-hon]

If no one volunteers by the end of this week(5/24), I'll take the challenge.