Refresh @Nested objects

[dain]

@Nested objects are only fetched once and used for all future requests. This makes it difficult to use immutable stat objects that are refreshed periodically. I would be nice if jmxutils supported fetching the object each time. This should be optional since fetching the object for every mbean attribute request could be very expensive. Additionally, it would be nice to jmxutils could cache the objects for a configurable period.