THis is currently done by sch_demux_send() (via demux_stream_send_to_dst()),
sch_enc_send() (via enc_send_to_dst()), and sch_dec_send() (via
dec_send_to_dst()), but not by sch_filter_send().
Implement the same queue-closing logic for them. The main benefit here is that
this will allow them to mark downstream inputs as send-done (in addition
to received-done), which is useful for a following commit.
d43fd5b332