etcdserver: save PeerURLs and Others into two different keys

for #1233 

i haven't cleaned up the code and tests. i wanna some feedback on this approach.
This is from https://github.com/coreos/etcd/issues/1266. We are moving all of our language to say "members" everywhere else so we might as well use this as a reason to cleanup this API.

/cc @jonboulle