fix gnmi set issues and simplify interface

Gnmi.Set() is simplified, leveraging ygot.Diff(). The Transport.Set() interface was clarified by replacing the empty interface{} parameter by the change.Payload struct containing the original and modified state both represented by a ygot.GoStruct

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.