merge r2895, r2896, r2926, r2927, r2928, r2930, and r2936: various proxy/fastcgi cache features and fixes: *) report about proxy/fastcgi_store and proxy/fastcgi_cache incompatibility *) delete useless r->cache->uses *) proxy_cache_use_stale/fastcgi_cache_use_stale updating *) inherit proxy_set_header, proxy_hide_header, and fastcgi_hide_header only if cache settings are similar *) add response file uniq while loading cold cache on demand