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>

feature/relayserver/relayserver.go

@@ -157,9 +157,7 @@ func (e *extension) consumeEventbusTopics(port int) {
 		select {
 		case <-e.disconnectFromBusCh:
 			return
-		case <-reqSub.Done():
-			// If reqSub is done, the eventClient has been closed, which is a
-			// signal to return.
+		case <-eventClient.Done():
 			return
 		case req := <-reqSub.Events():
 			if rs == nil {