store: use nextIndex where necessary
store: add etcd-index tests
etcdserver: add RaftIndex and RaftTerm
Merge pull request #1152 from jonboulle/1152_raft_headers

Missing X-Raft-Term & X-Raft-Index headers
Merge pull request #1153 from jonboulle/1153_etcd_index

X-Etcd-Index off by one
etcdserver: introduce the cluster and member

This introduces two new concepts: the cluster and the member.

Members are logical etcd instances that have a name, raft ID, and a list
of peer and client addresses.

A cluster is made up of a list of members.
Merge pull request #1147 from philips/add-name

introduce cluster store
etcdserver: publish self info when start