WIP 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.
@jonboulle i think current decision is:
1. id is immutable
2. PeerURLs could be changed explicitly through etcdctl or http endpoint
3. name and clientURLs are automatically updated each time etcd starts/restarts

/cc @xiangli-cmu @philips 

peerURLs are not that dynamic then..