- use a cachedb answer even if it's "expired" when serve-expired is yes

- use a cachedb answer even if it's "expired" when serve-expired is yes
  (patch from Jinmei Tatuya).
- trigger refetching of the answer in that case (this will bypass
  cachedb lookup)
- allow storing a 0-TTL answer from cachedb in the in-memory message
  cache when serve-expired is yes

git-svn-id: file:///svn/unbound/trunk@4353 be551aaa-1e26-0410-a405-d3ace91eadb9