Fixed HystrixContextScheduler to conform with RxJava Worker contract

[akarnokd]

Fixed issue in #354 by making HystrixContextScheduler conform with RxJava's Worker contract. See also #593.

To avoid code duplication, it uses the rx.internal.schedulers.ScheduledAction. I don't know how close Hystrix follows RxJava releases, but I'll make steps to have ScheduledAction part of the stable RxJava API eventually.

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[mattrjacobs]

This is great @akarnokd , thanks for the contribution!

The other piece of this issue is that a HystrixCommand has configuration to optionally perform a thread interruption on some unsubscribes. From what I can tell SchedulerAction.FutureCompleter already has a unsubscribe policy where thread interrupts happen if and only if the thread is different from the current. The policy I'm describing for Hystrix would be different from that, and determined at schedule-time.

I'm sure I could whip up a different hack for that - but is there a better way to make the thread-interruption policy more configurable?

[akarnokd]

I'm currently proposing a change for ScheduledAction in ReactiveX/RxJava#2579 which adds a system-property to enable/disable interruption globally. I'll add some interrupt-support there. But note if you get a ScheduledAction back from a wrapped worker, it is too late.

[mattrjacobs]

I'm merging this in. I'll open up a separate issue for the configurability of the thread-interrupt behavior. Then, depending on how RxJava and Hystrix release cycles line up, I'll likely just use the new behavior in RxJava (https://github.com/ReactiveX/RxJava/pull/2579/files).