Fix: relayd: viewer session trace chunk not released on detach

Fix: relayd: viewer session trace chunk not released on detach

The 'attach' command on a viewer session expects (asserts) the trace
chunk of the viewer session to be NULL. This is reasonable as there is
no reason to hold a reference to a trace chunk while no clients are
attached.

Release the reference to the trace chunk on detach. The relay
session's trace chunk will be re-sampled (copied) when the next client
attaches to the viewer session.

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