Task improvements

[alexandru]

Some task improvements:

• For Callback, removed the popBeforeCall and async builders, because they were confusing
• Instead Callback now has asyncOnSuccess and asyncOnError specified as extension methods (not methods on the class)
• Task.create was changed to be safe, in that users are protected from stack-overflows on usage of onSuccess - this isn't always a good solution for performance reasons, but users knowing what they are doing always have unsafeCreate
• Optimized chooseFirstOfList and ensured its stack safe - as part of it the implementation of CompositeCancelable was replaced with one that uses an internal mutable HashSet
• Optimized TestScheduler and as a result some tests are running dramatically faster