timeoutFirstOrDefault
If the upstream emits no first event within millis milliseconds it will emit the event returned by default followed by all later emissions from the upstream.
If the upstream emits no first event within duration it will emit the event returned by default followed by all later emissions from the upstream.
If the upstream emits no first event within millis milliseconds it will emit the event default followed by all later emissions from the upstream.
If the upstream emits no first event within duration it will emit the event default followed by all later emissions from the upstream.