raft: remove unused raftpb.LastIndex
Merge pull request #1085 from unihorn/127

raft: remove unused raftpb.LastIndex
etcdserver: introduce Server interface

This changes etcdserver.Server to an interface, with the former Server
(now "EtcdServer") becoming the canonical/production implementation.
This will facilitate better testing of the http server et al with mock
implementations of the interface.
It also more clearly defines the boundary for users of the Server.
Merge pull request #1061 from jonboulle/server_interface

etcdserver: introduce Server interface
etcdserver/etcdhttp: switch to using http.ServeMux