control/controlclient: proactively close TLS connection after /key fetch

When using Noise.

Updates #3488

Change-Id: I1049963763075a15b72fd8065dcf44a9cf37975f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f