actions: introduce stop session action

This patch introduces the API for the "stop session" action.

A stop session action is created using the
lttng_action_stop_session_create function.  It is mandatory to set a
session name using lttng_action_stop_session_set_session_name before
using the action in a trigger.

The patch adds the code for serializing the action and deserializing it
on the sessiond side, but not the code for executing it.

Change-Id: Ie00d744340f85f15a333680cf86d3882bd612d1a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>