Do not cache responses that do not have an explicit expiration time
(s-maxage or max-age Cache-Control directives or Expires header) or a
validator (ETag or Last-Modified headers) anymore, as suggested in
RFC 7234#3.
The TX_FLAG_IGNORE flag is used instead of the TX_FLAG_CACHEABLE so as
not to change the behavior of the checkcache option.
Res.cache_hit sample fetch returns a boolean which is true when the HTTP
response was built out of a cache. The cache's name is returned by the
res.cache_name sample_fetch.
This resolves GitHub issue #900.