store the upstream states before X-Accel-Redirect to a next upstream

src/http/ngx_http_request.h

diff --git a/src/http/ngx_http_request.h b/src/http/ngx_http_request.h
index 939e169..aac828a 100644
--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -329,6 +329,8 @@
     ngx_http_cache_t                 *cache;
 
     ngx_http_upstream_t              *upstream;
+    ngx_array_t                      *upstream_states;
+                                         /* of ngx_http_upstream_state_t */
 
     ngx_pool_t                       *pool;
     ngx_buf_t                        *header_in;