Resolve "The key for more complex paths in the map containing information of update events should be improved"

This MR changes the key for more complex paths (used within a subscribe event) from: qkd_node/qkd_links/qkd_link" key:{key:"qkdl_id" value:"acde2729-3544-41c3-88cb-9a9b82efeff0"/qkdl_status/ to /qkd_link[qkdl_id=acde2729-3544-41c3-88cb-9a9b82efeff0]/qkdl_status

Description

Using ygot.PathToString instead of our current own method for path to string parsing, results in a more manageable key within the event's pathAndValues map

Related Issue

#326

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.