* Revisit the algorithm used by getRRsetTrustLevel() in message_entry.cc.
* Implement dump/load/resize interfaces of rrset/message/recursor cache.
* Once LRU hash table is implemented, it should be used by message/rrset cache.
* Once the hash/lrulist related files in /lib/nsas is moved to seperated
  folder, the code of recursor cache has to be updated.
* Set proper AD flags once DNSSEC is supported by the cache.
* When the message or rrset entry has expired, it should be removed
  from the cache, or just moved to the head of LRU list, so that it
  can removed first.
* Make resolver cache be smart to refetch the messages that are about
  to expire.
* When the rrset beging updated is an NS rrset, NSAS should be updated
  together.
* Share the NXDOMAIN info between different type queries. current implementation
  can only cache for the type that user queried, for example, if user query A
  record of a.example. and the server replied with NXDOMAIN, this should be
  cached for all the types queries of a.example.
* Add the interfaces for resizing and serialization (loading and dumping) to
  cache.