DNSSEC: signatures refreshing
Included changes:
- The signatures are now refreshed (signature_lifetime / 10) seconds before their expiration. The default signature lifetime is 30 days, therefore the signatures are refreshed 3 days before their expiration.
- The parameter 'expires_at' in signing functions was renamed to 'refresh_at', as the name was misleading.
- The signing policy structure was cleaned and helper functions were added.
- DNSSEC event logging was changed from relative to absolute value, because the intervals are much longer now.