adjusting: implemented reverse tree for looking up additionals changed in update

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 list_t
of pointers to zone nodes, for which it actually is an addiitonal

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