Fix: Try loading classes using the thread context class loader

LTTngAgent should load classes using the current thread context's
class loader and then try using the system class loader if a
ClassNotFoundException is thrown.

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