Session consumed size notification

Add the support for notifications about the total amount of trace data
consumed for a session. The user can register itself to be notified when
a session has consumed more than a threshold. This sums the data for all
channels in a session.

For the review: part of this code was written by Jérémie, but it was on
top of my development branch with major updates on my early work with
notifications, so I had to squash it because it made no sense to keep
Jérémie's code separate.

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