cache: Rewrite to cache only on download (#7575)

This will allow cache to consistently work for
server and gateways. Range GET requests will
be cached in the background after the request
is served from the backend.

Fixes: #7458, #7573, #6265, #6630

cmd/bucket-handlers-listobjects.go

@@ -100,9 +100,7 @@ func (api objectAPIHandlers) ListObjectsV2Handler(w http.ResponseWriter, r *http
 	}
 
 	listObjectsV2 := objectAPI.ListObjectsV2
-	if api.CacheAPI() != nil {
-		listObjectsV2 = api.CacheAPI().ListObjectsV2
-	}
+
 	// Inititate a list objects operation based on the input params.
 	// On success would return back ListObjectsInfo object to be
 	// marshaled into S3 compatible XML header.
@@ -179,9 +177,6 @@ func (api objectAPIHandlers) ListObjectsV1Handler(w http.ResponseWriter, r *http
 	}
 
 	listObjects := objectAPI.ListObjects
-	if api.CacheAPI() != nil {
-		listObjects = api.CacheAPI().ListObjects
-	}
 
 	// Inititate a list objects operation based on the input params.
 	// On success would return back ListObjectsInfo object to be