Add support for API version 6

NGINX Plus API versions 6 introduces:

The [/stream/limit_conns/](https://nginx.org/en/docs/http/ngx_http_api_module.html#stream_limit_conns_) 
The [/http/limit_conns/](https://nginx.org/en/docs/http/ngx_http_api_module.html#http_limit_conns_)
The [/http/limit_reqs/](https://nginx.org/en/docs/http/ngx_http_api_module.html#http_limit_reqs_)