GUACAMOLE-393: Add AuthenticationProvider shutdown hook.

guacamole-ext/src/main/java/org/apache/guacamole/net/auth/AuthenticationProvider.java

@@ -167,5 +167,12 @@ public interface AuthenticationProvider {
     UserContext updateUserContext(UserContext context,
             AuthenticatedUser authenticatedUser,
             Credentials credentials) throws GuacamoleException;
+
+    /**
+     * Frees all resources associated with this AuthenticationProvider. This
+     * function will be automatically invoked when the Guacamole server is
+     * shutting down.
+     */
+    void shutdown();
     
 }

guacamole-ext/src/main/java/org/apache/guacamole/net/auth/simple/SimpleAuthenticationProvider.java

@@ -260,4 +260,9 @@ public abstract class SimpleAuthenticationProvider
         
     }
 
+    @Override
+    public void shutdown() {
+        // Do nothing
+    }
+
 }