In DNS::Resolve, they could be deleted once from where they were
stored in the nb_dns_info cookie and once again from where they
were stored in the DNS_Mgr::requests list. Before commit
bd9c937236, they were only deleted
from the requests list, so this commit reverts to that behavior
without any leaks being reported by the core/leaks tests.
Some of the changes only clean up at termination to make perftools
happt, but there were some "real" leaks as well.
This fixes all DNS leaks I could reproducem, including most likely
what's reported in #534. Closing #534.
I'm also adding a new btest subdir core/leaks with tests requiring
perftools support. These don't compare against base lines but abort
whenever perftools reports a leak (with stack information to track it
down). Right now, these are passing.
