The flag means that the process function might not complete
synchronously.

We can use this knowledge to improve the adapter. In sync mode we
can pull before scheduling the converter. In async mode we need to
schedule the follower after the converter to get the data ready for
the next iteration.

We can also improve the stream when it is operating async to schedule
a process call when a new buffer is ready to be filled.

This reduces a cycle latency from alsa and improves latency in
pulse.