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.
the reason is to make user feel flexible and safe at the same time.
PeerURLs could only be updated explicitly, so user will take care of its change. Or if user reboot all machines with new PeerURLs, the machines cannot build the cluster again.
The other information is not that critical and will not hurt the running of the cluster, so it is updatable each time reboot.