Fix: don't perform an automatic session rotation in snapshot mode

Fix: don't perform an automatic session rotation in snapshot mode

An automatic session rotation is performed on destruction if more than
one trace chunk was created during the lifetime of a session. This is
not a good criterion to use in no-output/snapshot mode as multiple
trace chunks may have been created during a session's lifetime.

The "output_traces" flag is checked explicitly to prevent an automatic
session rotation from happenning on destroy.

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