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/management_test.go

@@ -127,6 +127,7 @@ var _ = Describe("Management service", func() {
 				actualTURN := resp.WiretrusteeConfig.Turns[0]
 				Expect(len(actualTURN.User) > 0).To(BeTrue())
 				Expect(actualTURN.HostConfig).To(BeEquivalentTo(expectedTRUNHost))
+				Expect(len(resp.NetworkMap.OfflinePeers) == 0).To(BeTrue())
 			})
 		})