clean up unnecessary code in qpcache

clean up unnecessary code in qpcache

some code was left in the cache database implementation after
it was separated from the zone database, and can be cleaned up
and refactored now:

- the DNS_SLABHEADERATTR_IGNORE flag is never set in the cache
- support for loading the cache from was removed, but the add()
  function still had a 'loading' flag that's always false
- two different macros were used for checking the
  DNS_SLABHEADERATTR_NONEXISTENT flag - EXISTS() and NONEXISTENT().
  it's clearer to just use EXISTS().
- the cache doesn't support versions, so it isn't necessary to
  walk down the 'down' pointer chain when iterating through the
  cache or looking for a header to update.  'down' now only points
  to records that are deleted from the cache but have not yet been
  purged from memory. this allows us to simplify both the iterator
  and the add() function.