Fix: perform a rotation to a null trace chunk on session destruction

Fix: perform a rotation to a null trace chunk on session destruction

In order to guarantee the consumption of all data by the time a
session's destruction completes, a rotation to a "null" trace chunk is
performed as part of a session's destruction.

This ensures that a session is only reclaimed when all of its
streams (on both the consumers and the relay daemon) have been
destroyed.

The notion of a "quiet" rotation is introduced in order to re-use the
session rotation infrastructure, but without the rotation being
visible to external users through either the notification system or a
rename of the trace output folder.

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