Turn NetVST plugins OFF before quitting NetVSTHost

NetVSTHost acts as a port listener (server); the NetVST plugins NetFilter and NetSynth are its clients.

It's safe for a client to shut down its end of the socket connection; this is what happens when you turn the plugin's "on/off" setting to OFF. (It also happens automatically if the plugin instance is destroyed by the DAW.)

It is NOT safe for the server to shut down its end of the socket connection while clients are still sending/receiving data. This can cause the DAW to hang or crash.

To avoid this, always switch OFF all NetFilter/NetSynth plugin instances BEFORE quitting the NetVSTHost program they're connected to.