Don't compute result of Get Operation if subscriber unsubscribed

[artem-zinnatullin]

Currently we use OnSubscribeExecuteAsBlocking class that executes PreparedOperation anyway and then notifies Subscriber about result if it's not unsubscribed at this time.

It's okay for Put and Delete Operations because they should happen even if subscriber already unsubscribed, but we can skip it for Get Operation to prevent uneeded computations.

[Reagankm]

I'd like to work on this

[nikitin-da]

@Reagankm thank you! But we should make it over rxjava2 migration and check if emitter is not disposed here. Otherwise we will meet conflict =(