HADOOP-10376 added a new feature to register handlers for refreshing daemon configurations, because name node properties can be reconfigured without restarting the daemon. This can be a very useful generic interface, but so far no real handlers have been registered using this interface. I added a new 'list' handler to list all registered handlers. My plan is to add more handlers in the future using this interface.
Another minor fix is return a more explicit error message to the client if a handler is not registered. (It is currently logged at namenode side, but client side only gets a "Failed to get response." message without knowing why)