REORG/MEDIUM: channel: only use chn_prod / chn_cons to find stream-interfaces

REORG/MEDIUM: channel: only use chn_prod / chn_cons to find stream-interfaces

The purpose of these two macros will be to pass via the session to
find the relevant stream interfaces so that we don't need to store
the ->cons nor ->prod pointers anymore. Currently they're only defined
so that all references could be removed.

Note that many places need a second pass of clean up so that we don't
have any chn_prod(&s->req) anymore and only &s->si[0] instead, and
conversely for the 3 other cases.