pulseaudio: fix memory leak

This leak is due to memory allocated by pa_context_subscribe()
and not free as expected.
This function returns a pointer on a pa_operation structure.
This patch calls pa_operation_unref() on it when pa context is
terminated and when PulseAudio class is deleted.
This patch also improve the mainloop destroying (to prevent memory leak)
by using aa true RAII concept with a std::unique_ptr.

Change-Id: I8a27fd1672b7952f0fe1c944712ab6955cedfa97
Tuleap: #412