stream: Add an ASYNC flag for when we forward work in process()

In cases such as module-loopback, we don't actually consume queued buffers in the process callback, but chain via a trigger to another stream. In such a case, when we return from the process callback, the next buffer might want to be queued up, which means we should have a minimum of 2 buffers at hand.

We add a PW_STREAM_FLAG_ASYNC flag to inform PipeWire that this is the case.