Pass link to plugin message handlers

For master plugins a message can be received from any of the clients
corrently connected to the master.  In order for plugins to be able to
handle link specific requests they need a reference to the link messages
was received from.  Add link as the third argument to plugin message
handlers.

packages/lib/plugin.js

@@ -530,7 +530,7 @@ function attachPluginMessages(link, plugin) {
 				messageFormat.attach(link);
 
 			} else {
-				messageFormat.attach(link, async (message, format) => await plugin[handler](message, format));
+				messageFormat.attach(link, async (message, format) => await plugin[handler](message, format, link));
 			}
 
 		} catch (err) {