* @todo #32:30min Cancellation support.
* For now downstream and down downstream cancellation is not supported,
* but we definitely need to have it.
* @todo #32:30min Downstream emission subscription.
* We need to be aware of the fact that downstream switch doesn't happen instantly.
* For now, implementation does not rely on that.
* @todo #32:30min Full reactive streams tck compatibility.
* The processor implementation should be verified by reactive streams tck in order to ensure
* specification compatibility.