zone-load: fixed crash when loading non-existing zone file

- refactored the zone-loader a bit, as reading from zone file
  creates only contents and the zone is created separately
- refactored zone contents creating function, as it is always used in a
  way where an apex node is created and inserted into contents
- crash occured because created zone bound configuration and if parsing
  failed, the configuration was freed with the zone and also with the
  caller