etcdserver: correct X-Etcd-Index header

This adds an EtcdIndex field to store.Event and uses that as the header
instead of the node's modifiedIndex. To facilitate this in a non-racy
way, we set the EtcdIndex while holding the lock.
Merge pull request #1082 from jonboulle/1082_x_etcd_index

X-Etcd-Index returns the node's modifiedIndex