Improve logging of MaxDepthExceededException 

This makes small changes to how EhcacheSizeOfEngine logs MaxDepthExceededExceptions.

Firstly, it reduces the logging of the keys and values from WARN to DEBUG. This makes sense given that the warning relates to the fact that the exception has occurred, but logging of the keys and values can be extremely verbose, and is useful only for debugging the problem. When both are logged at WARN, then your only option is to get all of it, or none of it.

The second aspect of the change is to move the logging into a protected method, so that subclasses can more easily override this behaviour if they choose.