Major changes to zone database.

Each zone now holds a linked list of all its nodes while these
  are at the same time in the main zone data structure.
  => added pointer to the zone apex to zdb_zone
  => added function zdb_add_zone() to add a zone in ldns_zone
     format to the database.
Note: zdb_insert() does nothing with the given node, so it should
      be used only with nodes already in the list.
TODO: Some way to add a new node to the zone should be provided.

Added owner name and pointers to next and previous item in list
  to the zone node structure + added function zn_owner() for
  getting the node's owner.

Changed API of zone-data-structure: zds_insert() now has only 2
  arguments - the owner is in the node.