Add filter change notification mechanism to the Java agent

Java applications can now register to receive notifications of event
filtering rules being changed in the tracing session. This can be
used to implement application-specific filtering on the Java side,
to reduce the amount of events sent through JNI.

To do so, they need to implement a IFilterChangeListener and register
it to the FilterChangeNotifier. The listener's callbacks will
be invoked by the LTTng agent when the tracing session(s) change.

A new example file is provided to demo this usage.

Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>