Channel rotate pipe between sessiond and the consumers

This new pipe is used by the consumers to inform the session daemon (in
the rotation_thread) that it has finished the rotation of a channel. In
this patch, we only setup the pipe between the daemons, it is not yet in
use.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>