sessiond: setup event notifier group for registering app

Create a pipe for each application and setup an event notifier group
associated with that pipe. Transfer the write side to the app, and
transfer the read side to the notification thread as an application
event source ([...]_ADD_TRACER_EVENT_SOURCE)

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3e4aab84e3270ddef1f50f72f946a4d80b3f36e0
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479