DW:

DW:
 - Rewrite of much of refreshCheck().

   This was prompted by a bug report that we didn't properly handle the
   must-revalidate directive.  Indeed, we always caused a refresh
   for must-revalidate.   The spec says must-revalidate only takes effect
   for stale responses.

   I find the spec to be unclear on how to handle must-revalidate without
   an explicit expiration time.  Currently, if we have a response
   that has must-revalidate, no expires, but the last-modified factor
   rule says the reponse is fresh, then we don't force a validation.

   Also added support for the max-stale request directive.

   I changed some of the internal accounting to use status codes.  In the
   future it may be useful to have a "refresh.log" file so people can
   figure out why a particular request got validated, or didn't.