Sync
An Observer.Sync is an Observer that signals demand
to upstream synchronously (i.e. the upstream observable doesn't need to
wait on a Future in order to decide whether to send the next event
or not).
An Observer.Sync is an Observer that signals demand
to upstream synchronously (i.e. the upstream observable doesn't need to
wait on a Future in order to decide whether to send the next event
or not).
Can be used for optimizations.