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.
we should put it into /_etcd/members and call this a new API. The old /_etcd/machines format is incompatible:

```
etcd=http%3A%2F%2F127.0.0.1%3A4001\u0026raft=http%3A%2F%2F127.0.0.1%3A7001
```