merge r2895, r2896, r2926, r2927, r2928, r2930, and r2936:

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