* 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.

