Live: send the number of streams in list_sessions

This allows the viewer to quickly know if a session is really active or
just created and idle. Before this patch, we had to attach to a session
to discover the number of streams active in that session.

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