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
Are we OK with this prefix method being the ABI in the WAL file moving forward? It seems like a matter of the internal API not being in a good enough state but the general idea of a prefix is fine.