reintroduce channels

Bring back the channel object. Making a node and port on the client side
was rather awkward because of the async nature of many methods. It feels
better to have a specific communication channel object to interface with
a server side port.
Use port activate/deactivate to start/stop streams
Remove links from the ports. We let other objects install a callback on
the port to receive and route buffers.

pinos/client/stream.h

@@ -59,6 +59,7 @@ typedef enum {
 typedef enum {
   PINOS_STREAM_MODE_SOCKET = 0,
   PINOS_STREAM_MODE_BUFFER = 1,
+  PINOS_STREAM_MODE_RINGBUFFER = 2,
 } PinosStreamMode;
 
 /**