This change delays the first flush in the /watch-ipn-bus/ handler
until after the watcher has been successfully installed on the IPN
bus. It does this by adding a new onWatchAdded callback to
LocalBackend.WatchNotifications().
Without this, the endpoint returns a 200 almost immediatly, and
only then installs a watcher for IPN events. This means there's a
small window where events could be missed by clients after calling
WatchIPNBus().
Fixestailscale/corp#8594.
Signed-off-by: salman <salman@tailscale.com>