feature,ipn/ipnlocal,wgengine: improve how eventbus shutdown is handled (#17156)

Instead of waiting for a designated subscription to close as a canary for the
bus being stopped, use the bus Client's own signal for closure added in #17118.

Updates #cleanup

Change-Id: I384ea39f3f1f6a030a6282356f7b5bdcdf8d7102
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/expiry.go

@@ -68,15 +68,13 @@ func newExpiryManager(logf logger.Logf, bus *eventbus.Bus) *expiryManager {
 // [eventbus.Subscriber]'s and passes them to their related handler. Events are
 // always handled in the order they are received, i.e. the next event is not
 // read until the previous event's handler has returned. It returns when the
-// [controlclient.ControlTime] subscriber is closed, which is interpreted to be the
-// same as the [eventbus.Client] closing ([eventbus.Subscribers] are either
-// all open or all closed).
+// [eventbus.Client] is closed.
 func (em *expiryManager) consumeEventbusTopics() {
 	defer close(em.subsDoneCh)
 
 	for {
 		select {
-		case <-em.controlTimeSub.Done():
+		case <-em.eventClient.Done():
 			return
 		case time := <-em.controlTimeSub.Events():
 			em.onControlTime(time.Value)