MEDIUM: dynbuf/stream: do not allocate the buffers in the callback

MEDIUM: dynbuf/stream: do not allocate the buffers in the callback

One of the problematic designs with the buffer_wait mechanism is that
the callbacks pre-allocate the buffers and stay in the run queue for
a while, resulting in all of the few buffers being assigned to waiting
tasks instead of being all available to one task that needs them all at
once.

Here we simply stop doing this, the callback clears the waiting flags
and wakes the task up so that it has a chance of still finding some
buffers.