etcdserver/etcdhttp: store location adjustment

Detailed adjustment:
/_etcd/machines/* -> /0/members/*
/* -> /1/*

And it keeps key path returned to user the same as before.
etcdserver: const XXXDir -> StoreXXXPrefix

and code clean
etcdhttp: parseRequest -> parseKeyRequest
etcdhttp: writeEvent -> writeKeyEvent
Merge pull request #1335 from unihorn/174

etcdserver/etcdhttp: store location adjustment