control/controlclient, tailcfg: add Node.Expired field, set for expired nodes

Nodes that are expired, taking into account the time delta calculated
from MapResponse.ControlTime have the newly-added Expired boolean set.
For additional defense-in-depth, also replicate what control does and
clear the Endpoints and DERP fields, and additionally set the node key
to a bogus value.

Updates #6932

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ia2bd6b56064416feee28aef5699ca7090940662a

tailcfg/tailcfg_test.go

@@ -334,7 +334,7 @@ func TestNodeEqual(t *testing.T) {
 		"Capabilities",
 		"UnsignedPeerAPIOnly",
 		"ComputedName", "computedHostIfDifferent", "ComputedNameWithHost",
-		"DataPlaneAuditLogID",
+		"DataPlaneAuditLogID", "Expired",
 	}
 	if have := fieldsOf(reflect.TypeOf(Node{})); !reflect.DeepEqual(have, nodeHandles) {
 		t.Errorf("Node.Equal check might be out of sync\nfields: %q\nhandled: %q\n",
@@ -514,6 +514,11 @@ func TestNodeEqual(t *testing.T) {
 			&Node{},
 			false,
 		},
+		{
+			&Node{Expired: true},
+			&Node{},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)