Support DNS Service (SRV) records to get the port numbers of upstream servers via DNS

[hjr3]

This would allow users to add servers with only a dns record instead of specifying an ip addrses and port. This may be required in order to do https://github.com/hjr3/alacrity/issues/49

[hjr3]

See https://www.nginx.com/blog/dns-service-discovery-nginx-plus/ for some details.

[yanns]

for info, support in hyper: https://github.com/hyperium/hyper/blob/master/src/client/dns.rs