etcdserver: allow zero kv index for cluster upgrade

If a user upgrades etcd from 2.3.x to 3.0 and shutdown the
cluster immediately without triggering any new backend writes,
then the consistent index in backend would be zero.

The user cannot restart etcdserver due to today's strick index
match checking. We now have to lose this a bit for this case.

etcdserver/server.go

@@ -408,8 +408,13 @@ func NewServer(cfg *ServerConfig) (srv *EtcdServer, err error) {
 	srv.kv = mvcc.New(srv.be, srv.lessor, &srv.consistIndex)
 	if beExist {
 		kvindex := srv.kv.ConsistentIndex()
+		// TODO: remove kvindex != 0 checking when we do not expect users to upgrade
+		// etcd from pre-3.0 release.
 		if snapshot != nil && kvindex < snapshot.Metadata.Index {
-			return nil, fmt.Errorf("database file (%v index %d) does not match with snapshot (index %d).", bepath, kvindex, snapshot.Metadata.Index)
+			if kvindex != 0 {
+				return nil, fmt.Errorf("database file (%v index %d) does not match with snapshot (index %d).", bepath, kvindex, snapshot.Metadata.Index)
+			}
+			plog.Warningf("consistent index never saved (snapshot index=%d)", snapshot.Metadata.Index)
 		}
 	}
 	srv.consistIndex.setConsistentIndex(srv.kv.ConsistentIndex())