ipn,tka: improve Tailnet Lock logs

* Refer to "tailnet-lock" instead of "network-lock" in log messages
* Log keys as `tlpub:<hex>` rather than as Go structs

Updates tailscale/corp#39455
Updates tailscale/corp#37904

Change-Id: I644407d1eda029ee11027bcc949897aa4ba52787
Signed-off-by: Alex Chan <alexc@tailscale.com>

tka/builder.go

@@ -67,11 +67,11 @@ func (b *UpdateBuilder) AddKey(key Key) error {
 	}
 
 	if _, err := b.state.GetKey(keyID); err == nil {
-		return fmt.Errorf("cannot add key %v: already exists", key)
+		return fmt.Errorf("cannot add key tlpub:%x: already exists", key.Public)
 	}
 
 	if len(b.state.Keys) >= maxKeys {
-		return fmt.Errorf("cannot add key %v: maximum number of keys reached", key)
+		return fmt.Errorf("cannot add key tlpub:%x: maximum number of keys reached", key.Public)
 	}
 
 	return b.mkUpdate(AUM{MessageKind: AUMAddKey, Key: &key})