ESDEV-525 Improve reverse proxy flushing (part2)

Implement checking if category flushing is really needed.
Flushing only when sortable fields has changed.
ESDEV-525 Improve reverse proxy flushing (part2)

Moving saving of sorting fields values on load to separate method.
ESDEV-525 Improve reverse proxy flushing (part2)

Fixing problems when no sorting fields are set.