MEDIUM: mux-h1: allocate without queuing when retrying

MEDIUM: mux-h1: allocate without queuing when retrying

Now when trying to allocate a buffer, we can check if we've been notified
of availability via the callback, in which case we should not consult the
queue, or if we're doing a first allocation and check the queue. At this
point it still doesn't change much since the stream still doesn't make use
of it but some progress is expected.