Fix: lttng-live: send HUP reply when per-PID streams are gone

Currently, the "HUP" reply (hung up) is only sent when a session destroy
is performed and all data has been sent. However, with per-pid buffers,
application can come and go during the tracing session, so we need to
send the HUP reply for each individual stream that is gone.

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