Implement a kind of "Snapshot MBeanServerConnection".

The idea is that this is an object that wraps an existing MBeanServerConnection
and adds caching to it, as follows:

* The first time an attribute is called in a given MBean, the result is cached.
  Every subsequent time getAttribute is called for that attribute the cached
  result is returned.

* Before every VMPanel.update() or when the Refresh button in the Attributes
  table is pressed down the attributes cache is flushed. Then any subsequent
  call to getAttribute will retrieve all the values for the attributes that
  are known to the cache.

* The attributes cache uses a learning approach and only attributes that are
  in the cache will be retrieved between two subsequent updates.