This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.
Used this tool to update package imports:
https://github.com/KSubedi/gomove
Trying to decouple the v2 client from SRV code. Can't move
into discovery/ since that creates a circular dependency. So,
give up and move all the SRV code into a new package.
Based on code from discovery/srv.go. The returns the target as DNS
returns it. In the case of SSL, certs are tied to the hostname and not
the IP address generally.
Solves #2547
