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
Yeah that makes sense. Re: wrapping Store, arguably this would be an opportunity to create a more sensible interface, since we will be swapping out the underlying store soon anyway... But this may not be the time. This approach doesn't prevent that in future afaict.