MINOR: mux: Add a destroy() method.

Add a new method to muxes, destroy(), that is responsible for destroying
the mux and the associated connection, to be used for server connections.
diff --git a/include/types/connection.h b/include/types/connection.h
index 60036d6..2ed39f6 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -327,6 +327,7 @@
 	int (*subscribe)(struct conn_stream *cs, int event_type, void *param); /* Subscribe to events, such as "being able to send" */
 	int (*unsubscribe)(struct conn_stream *cs, int event_type, void *param); /* Unsubscribe to events */
 	int (*avail_streams)(struct connection *conn); /* Returns the number of streams still available for a connection */
+	void (*destroy)(struct connection *conn); /* Let the mux know one of its users left, so it may have to disappear */
 	unsigned int flags;                           /* some flags characterizing the mux's capabilities (MX_FL_*) */
 	char name[8];                                 /* mux layer name, zero-terminated */
 };