Support for keepalive_timeout in upstream block

[jils2013]

http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive_timeout
This directive appeared in version 1.15.3.
Sets a timeout during which an idle keepalive connection to an upstream server will stay open.

any plan to support this directive in ngx.balancer.set_timeouts()? https://github.com/openresty/lua-resty-core/blob/master/lib/ngx/balancer.md#set_timeouts

[doujiang24]

yeah, it should be useful, patches welcome :)