Support cancellation for async methods

Currently azure java libraries are depend on rxjava1 where `Completable` and `Observable` in `*Async` method results do not support any cancellation methods and offer only `Subscription#unsubscribe`. In rxjava2 exists [Disposable#dispose](http://reactivex.io/RxJava/javadoc/io/reactivex/disposables/Disposable.html) which allows to terminate async execution.

It makes impossible to support cooperative cancellation for such methods, e.g. in [Kotlin coroutines](https://github.com/Kotlin/kotlinx.coroutines/blob/master/docs/cancellation-and-timeouts.md#cancellation-and-timeouts) which has [rxjava2 extensions](https://github.com/Kotlin/kotlinx.coroutines/blob/master/reactive/kotlinx-coroutines-rx2/README.md).