Add a save API to lttng-ctl

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add session configuration save feature to session daemon

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add a save command based on the save API to the lttng client

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