Consumer rotate a channel

This command is sent from the session daemon for each channel in a
session. When the consumer receives the command, it stores the position
at which we need to rotate the stream. If a stream is ready to be
rotated, we perform the rotation immediately.

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