MEDIUM: stream: No longer use TASK_F_UEVT* to shut a stream down

MEDIUM: stream: No longer use TASK_F_UEVT* to shut a stream down

Thanks to the previous patch, it is now possible to explicitly rely on
stream's events to shut it down. The right event is set in
stream_shutdown(), before waking up the stream, via an atomic operation. In
process_stream(), this event will be handled as expected.

Thus, TASK_F_UEVT* are no longer used, but not removed since still usable
for other tasks.

This patch depends on "MEDIUM: stream: Map task wake up reasons to dedicated
stream events".