Explicitly stop the session on lttng destroy

This changes the default behavior of the "destroy" command and API: now
it implicitely stops the tracing and waits for the data to be available
on disk or the network. Like the "stop" command and API, a no_wait
option is available to skip the waiting and destroy directly the
session.

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