HTTP Compliance: add appropriate Warnings if serving a stale hit.

HTTP Compliance: add appropriate Warnings if serving a stale hit.

Per RFC 2616, we MUST add "110 Response is stale" Warning if serving a
stale reply for any reason, including configured overrides. We MUST add
"111 Revalidation failed" Warning if serving a stale reply because an
attempt to revalidate the response failed, due to an inability to reach
the server.

The patch adds a new stale_if_hit request flag, which is set in
refreshCheckHTTP() when entry freshness is calculated. refreshCheckHTTP()
is now called in offline mode, to set stale_if_hit properly. We check for
the offline mode before returning from refreshCheckHTTP() to preserve the
original logic.

refreshCheckHTTP() is no longer called for internal requests, to avoid
setting of stale_if_hit flag. It did not do anything important for
internal requests anyway.

Co-Advisor test cases:
    test_case/rfc2616/noSrv-hit-stale-max-age-req
    test_case/rfc2616/ccReqDirMsg-max-stale-warning