enable sendfile support again

There's a remaining issue fixed in mainline that's not fixed in the
current stable branch yet, but it doesn't apply unless HTTP/2 is being
used without encryption. Currently sendfile is only really used for the
backend proxy connections in practice due to TLS, and those are never
HTTP/2.

nginx/nginx.conf

@@ -16,8 +16,7 @@ http {
     charset utf-8;
     charset_types text/css text/plain text/xml application/atom+xml application/javascript;
 
-    # sendfile + AIO is buggy without fixes from nginx master
-    #sendfile on;
+    sendfile on;
     sendfile_max_chunk 512k;
     tcp_nopush on;
     keepalive_timeout 3m;