Fix: relayd live don't send incomplete stream list

Sample the "closed" flag with session lock held.
Also, if the session connection is closed while we attach to it, reply
with HUP, because there are risks of attaching to an incomplete session
(e.g. the metadata stream could be already closed).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>