Sessiond rotation thread

This thread is responsible to receive the notifications from the
consumers that a channel has finished its rotation and perform the
rename of the chunk ready to be processed by the client when all the
channels of a session have completed their rotation.

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