Simplify Metadata class

[roll]

Overview

For the v5 we improved our metadata model and added separation between the model and static descriptors. Even though it has been done Metadata class is still quite complex and hard to understand. We need to simplify it.

Implementation

• Completely split the metadata model and static descriptors validation (probably creating a separate python package - Experiment with the metadata mapping package in Python frictionlessdata.io#784)
• Review the dereferenced descriptors model (the metadata_descriptor_path/initial API); After the model/descriptor separation it will stop working anyway
• Simplify/improve the way we deal with default values (the defined API)