Allow keepalives on status port

A few years ago, #3684 changed the status port to use Unix sockets rather than TCP. As part of that change, `keepalive_timeout 0` was added. Sometime later, #4487 changed the status port back to TCP. However, keepalive was never re-enabled. In practice we're seeing behavior where nginx stops accepting GET or POST to the status port during graceful shutdown since new connections aren't allowed.

Author: rohansingh

rootfs/etc/nginx/template/nginx.tmpl

@@ -654,7 +654,6 @@ http {
  listen 127.0.0.1:{{ .StatusPort }};
  set $proxy_upstream_name "internal";
 
- keepalive_timeout 0;
  gzip off;
 
  access_log off;