allow background IAM load to speed up startup (#9796)

Also fix healthcheck handler to run success
only if object layer has initialized fully
for S3 API access call.

cmd/healthcheck-handler.go

@@ -37,7 +37,7 @@ func ReadinessCheckHandler(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithTimeout(ctx, globalAPIConfig.getReadyDeadline())
 	defer cancel()
 
-	if !objLayer.IsReady(ctx) {
+	if !objLayer.IsReady(ctx) && newObjectLayerFn() == nil {
 		writeResponse(w, http.StatusServiceUnavailable, nil, mimeNone)
 		return
 	}