upgraded the nginx core to 1.13.2 (some patches fail to apply though).

patches/nginx-1.13.2-upstream_pipelining.patch

@@ -0,0 +1,23 @@
+commit f9907b72a76a21ac5413187b83177a919475c75f
+Author: Yichun Zhang (agentzh) <agentzh@gmail.com>
+Date:   Wed Feb 10 16:05:08 2016 -0800
+
+    bugfix: upstream: keep sending request data after the first write attempt.
+    
+    See
+    http://mailman.nginx.org/pipermail/nginx-devel/2012-March/002040.html
+    for more details on the issue.
+
+diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
+index dbaa956..a25aaa2 100644
+--- a/src/http/ngx_http_upstream.c
++++ b/src/http/ngx_http_upstream.c
+@@ -2003,7 +2003,7 @@ ngx_http_upstream_send_request_handler(ngx_http_request_t *r,
+ 
+ #endif
+ 
+-    if (u->header_sent) {
++    if (u->request_body_sent) {
+         u->write_event_handler = ngx_http_upstream_dummy_handler;
+ 
+         (void) ngx_handle_write_event(c->write, 0);