Do Not Reschedule Callbacks

- Use `Immediate` so that we don't add scheduling overhead unless needed.
- Reuse the Hystrix thread if it is there. The computation of the callback can be part of the Hystrix thread.
- No one using Hystrix+Observable should be doing anything blocking on those callbacks.
Semaphore vs Thread Isolation

- only SEMAPHORE reject if SEMAPHORE isolation is being used
- unit test to assert that THREAD and SEMAPHORE isolation reject independently of each other
Merge pull request #238 from benjchristensen/semaphore-vs-thread-isolation

Fix for Semaphore vs Thread Isolation Bug