Fix: sessiond should not error on channel creation vs app exit

We should not report an error when creating a channel if the application
is exiting concurrently.

Also, remove an inappropriate assert() in ust_app_create_event_glb: it
is possible to have a channel lookup fail if channel/event creation
occurs concurrently with an application exit.

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