In case the server->serv_domain points to domain also when it is not the
last server used, ensure the reference to last_server is always reset.
Some records might reference the server_domain, but cannot ever become
last_server. Such as server=/example.com/#
Do proper check also for above case and do not delete used serv_domain
structure. Also do optimization to reuse common server domains and do
not create new entry to already existing one.
Do two step cleaning during nameservers cleanup stage. Should avoid any
invalid pointer present.
Resolves: rhbz#2186481
