feat(server): add UpdatePeer

The function is used to update peer info and rewrite conf file.

This is caused by coreos/etcd#639
I want to update the peer information to adopt new IP when joining with different IP.
Then I found it cannot load these information when restart because `commitIndex` in `conf` decides how many commits raft will load from the log when start, and I didn't update it.
So I need to add this function to update `conf` file to load correct information.