Use separate resolvers

[gfrankliu]

Instead of using system's /etc/resolv.conf, can we use different resolvers? nginx now allows to define own resolvers http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver but I am not sure if this module can use that.
This will allow us to query service discovery platform, e.g.: https://www.consul.io/docs/agent/dns.html
upstream servers can register against "consul", and "consul" can take care of health check.

Can you also add the dns SRV record support? We can then set the weight, port, etc directly in the DNS.