Mesh design and preparatory cleanup.

Mesh design and preparatory cleanup.
      - removed unused _node iterator value from rbtree_t. Takes up space.
      - iterator can handle querytargets state without a delegation point
        set, so that a priming(stub) subquery error can be handled.
      - iterator stores if it is priming or not.
      - log_query_info() neater logging.

git-svn-id: file:///svn/unbound/trunk@418 be551aaa-1e26-0410-a405-d3ace91eadb9