Live timer set up

Add a new option to create to enable the live_timer being --live at
session creation. A new API call is added also to create as session in
live mode like the snapshot feature.

This timer is responsible to check all streams of a live session and
make sure the viewer receives periodic updates even for low-throughput
channels.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>