public abstract class AsyncSubscriber<T>
implements org.reactivestreams.Subscriber<T>, java.lang.Runnable
AsyncSubscriber is an implementation of Reactive Streams `Subscriber`,
it runs asynchronously (on an Executor), requests one element
at a time, and invokes a user-defined method to process each element.
NOTE: The code below uses a lot of try-catches to show the reader where exceptions can be expected, and where they are forbidden.