There are three types of ports depeneding on references to them:
* ports that are referenced from jack and studio graphs
* ports that are referenced from jack and room graphs
* studio-room link ports that are referenced from studio and room graphs
The old approach was to destroy ports when removing references from jack graph.
Studio-room link ports were destroyed when removing the room.
However this caused double free when removing app ports that appear in jack and
room graphs.
The introduced implementation uses reference counting of graph references to ports.
Temporary references (like the one returned by port constructor and graph_get_port())
are not tracked.
Nedko Arnaudov