add peer store

This store is designed for node register.

It is used instead of `Peer` for etcdserver to send requests and provide machine infos.

It is necessary to have such a struct to fit: 1. dynamic change on nodes 2. save/load from log/snapshot
can we move this file to peers.go
this seems like a persistent layer of peers... it should be a very thin layer on top of the whatever store passed into etcdserver.