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 I find it is better to put in `parseRequest` when revisiting the code. `handleKeyWatch` does the trim job too, and they cannot be put into the same function. I will rename `parseRequst` -> `parseKeyRequest` to make it clear what happens inside. Sound good?