ipn/ipnlocal: bound how long cert fetchher checks for existing DNS records

It was supposed to be best effort but in some cases (macsys at least,
per @marwan-at-work) it hangs and exhausts the whole context.Context
deadline so we fail to make the SetDNS call to the server.

Updates #8067
Updates #3273 etc

Change-Id: Ie1f04abe9689951484748aecdeae312afbafdb0f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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