Support SRV records for cluster discovery

Currently the 'discover' option of serf supports mDNS, but this depends on multicast, and thus is not a viable solution in AWS and other unicast-only environments.

Our current work-around for this is to implement 'anchor nodes' for our cluster, which bootstrap the cluster and have SRV records associated with them.

When other nodes come up, they find the anchor nodes by SRV record and join their cluster.

This would be quite easy to add natively to serf, and I'd be fine with drawing up the PR myself if you think it's something that might get accepted.

IMHO, supported a form of service discovering that's very reliable and proven (DNS SRV) for environments that need it (can't use mDNS) is quite worthwhile.