HF26 release changes

All changes done in develop branch prepared for delivery into master as v1.26 release.

• Compatibility changes to PostgreSQL 12 (including performance optimizations specific to it)
• Package versioning is more stable (eliminated packages with dangling versions). Also python code has been adjusted for Python 3.8 needs.
• Improvements in setup to match PEP517 and PEP518 rules
• Optimization of update_notification_cache procedure
• Prerequisites for HAF based version (to be delivered soon, now it's present in develop-haf branch).