etcdserver/etcdhttp: store location adjustment

Detailed adjustment:
/_etcd/machines/* -> /0/members/*
/* -> /1/*

And it keeps key path returned to user the same as before.

fixes #1331
@jonboulle Not that crazy but not that good too imo. A separate Store will introduce new problems: 1. need extra struct when saving and loading the new store snapshot 2. may need new field on Request to indicate the apply to the new store 3. there exist multi-version Etcd-Index then.. 4. the separate solution may encourage more separate Stores in the future, which makes code logic complicated.