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>
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