Reduce and try to eliminate need to read the attribute DMR description in ModelControllerMBeanHelper

The TypeConverter stuff used by ModelControllerMBeanHelper relies on the full DMR description for an attribute in order to do the conversion. But getting that description is expensive relative to the amount of data that is extracted from it. Task is to figure out a lower overhead way, at least for non-OBJECT/LIST/PROPERTY attributes where all that's needed is AttributeDefinition.getType(). It's more complex for non-simple attributes, but those are less common.