Additionals reverse tree

the tree is like a key-value database, with the key being a domain name of a record which is an additional for a record in the zone while the name might be not in the zone; the value is a deduplicated dynarray of pointers to zone nodes, for which it actually is an additional

whenever in an incremental update a record is changed/added with the name of an additional, all the listed nodes must be re-adjusted (re-discovered additionals

previously, we always re-discovered additionals for whole zone