Convert the window pane (pty master side) fd over to use a bufferevent.

The evbuffer API is very similar to the existing tmux buffer API so this was
remarkably painless. Not many possible ways to do it, I suppose.

server.c

@@ -214,7 +214,6 @@ server_loop(void)
 	while (!server_should_shutdown()) {
 		server_update_socket();
 
-		server_window_prepare();
 		server_client_prepare();
 
 		event_loopexit(&tv);