Fix: ust-app: protect app socket protocol with lock

Fix: ust-app: protect app socket protocol with lock

Many threads can access the application socket (cmd handling thread and
application handling thread) concurrently. Therefore, we need to protect
it with a mutex.

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