We were eagerly doing a synchronous renewal of the cert while
trying to serve traffic. Instead of that, just do the cert
renewal in the background and continue serving traffic as long
as the cert is still valid.
This regressed in c1ecae13ab708cef90905085f87729974f6c339d when
we introduced ARI support and were trying to make the experience
of `tailscale cert` better. However, that ended up regressing
the experience for tsnet as it would not always doing the renewal
synchronously.
Fixes#9783
Signed-off-by: Maisem Ali <maisem@tailscale.com>
24f322bc43