Report offline peers to agents (#728)

The peer login expiration ACL check introduced in #714
filters out peers that are expired and agents receive a network map 
without that expired peers.
However, the agents should see those peers in status "Disconnected".

This PR extends the Agent <-> Management protocol 
by introducing a new field OfflinePeers
that contain expired peers. Agents keep track of those and display 
then just in the Status response.

management/server/network.go

@@ -23,10 +23,11 @@ const (
 )
 
 type NetworkMap struct {
-	Peers     []*Peer
-	Network   *Network
-	Routes    []*route.Route
-	DNSConfig nbdns.Config
+	Peers        []*Peer
+	Network      *Network
+	Routes       []*route.Route
+	DNSConfig    nbdns.Config
+	OfflinePeers []*Peer
 }
 
 type Network struct {