Metadata Handler methods should be provided the model

[ceharris]

A metadata handler should get a reference to the model object associated with the parent container node in produceValue, and a reference to the ViewEntity that represents the model object associated with the parent container node in consumeValue.

This would allow, for example, a metadata handler to produce a value that is based on some aspect of the model. For example, the properties of the model could be digested to produce an ETag-like value for the model node. Or a metadata value could be used to update/augment the model state in some way.