Rework how clients connect.

Add buffer flags. The idea is to make it possible to easily check when a
buffer contains control information that we need to parse to update the
port fields.
Make the client create remote nodes and ports and set up proxies for
them.
Make a port base class implementing most of the logic to pass buffers
locally and remotely.
Remove most code from stream.c, it's now in the port.
Make a portsink and portsrc that can write and read to/from any port. We
use these in the server to send and receive data.
Rework format negotiation. The final format is now sent in-line before
the data. The server will select a format on output ports.

pinos/tools/pinos-monitor.c

@@ -163,6 +163,7 @@ dump_port_info (PinosContext *c, const PinosPortInfo *info, gpointer user_data)
     g_print ("%c\tname: \"%s\"\n", MARK_CHANGE (0), info->name);
     print_properties (info->properties, MARK_CHANGE (1));
     print_formats ("possible formats", info->possible_formats, MARK_CHANGE (2));
+    print_formats ("format", info->format, MARK_CHANGE (3));
   }
 }