Defers creation of a single element source until there is demand.
Invokes the user supplied factory when the first downstream demand arrives, then emits the returned single value downstream and completes the stream.
Note that asynchronous boundaries (and other operators) in the stream may do pre-fetching which counter acts the laziness and will trigger the factory immediately.
Reactive Streams semantics
emits when there is downstream demand and the element factory has completed
completes after emitting the single element