Do the following tweaks for negative rdataset handling:
* rdataset->rdh_ttl is rdataset->ttl + addrdataset time
* During add32(), modify code so that if the header is negative, append
it at the end of the LRU list
* Don't modify need_headerupdate()
* During update_header(), if the header is negative:
+ If update_header() was called < 1 minute after addrdataset() [which
can be determined by rdataset->rdh_ttl vs. rdataset->ttl vs. now],
don't update the header.
+ Otherwise, bring the header to the front of the LRU list as
currently done.
----
With these changes, during a random subdomain attack, all these negative
answers will stick to the end of the LRU lists (where they'll be cleaned
out first) and if a negative answer is actually re-used after a minute,
then it'll be moved to the front of the LRU lists (i.e., it won't be
cleaned out).
We can also remove this second part if we want to be strict - i.e., just
let all negative answers get cleaned up first whether they have been
recently used or not.
Mukund