Retain UNL validators in purge

[pdp2121]

High Level Overview of Change

With current logic, if a validator has not participated in the consensus process for more than a week, it will be removed from the list. This led to an UNL validator, even though still in the UNL list, got removed by the logic.
This PR would:

• Retain inactive UNL validators that are still discoverable in the UNL list in the database, until it got removed from the list.
• Increase the purging period from one week to thirty days.

Resolves #395
Replaces #397

Type of Change

• 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 not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release