controlclient: log server key and URL

Turns out we never reliably log the control plane URL a client connects
to. Do it here, and include the server public key, which might
inadvertently tell us something interesting some day.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f