BUG/MEDIUM: cache: respect the request cache-control header

BUG/MEDIUM: cache: respect the request cache-control header

Till now if a client emitted a request featureing a cache-control header,
this one was not respected and a stale object could still be delievered.r
 This patch ensures that :
  - cache-control: no-cache disables retrieval from the cache but does
    not prevent the newly fetched object from being stored ;
  - cache-control: no-store can safely retrieve from the cache but prevents
    from storing any fetched object
  - cache-control: max-age/max-stale/min-fresh act like no-cache
  - pragma: no-cache acts like cache-control: no-cache.

This needs to be backported to 1.8.