Manager: allow requesting shutdown multiple times

It's fine to request a shutdown multiple times. This fixes a hard crash
due to a panic.

internal/manager/api_impl/api_impl.go

@@ -83,6 +83,11 @@ func (f *Flamenco) WaitForShutdown(ctx context.Context) bool {
 // requestShutdown closes the 'done' channel, signalling to callers of
 // WaitForShutdown() that a shutdown is requested.
 func (f *Flamenco) requestShutdown() {
+	defer func() {
+		// Recover the panic that happens when the channel is closed multiple times.
+		// Requesting a shutdown should be possible multiple times without panicing.
+		recover()
+	}()
 	close(f.done)
 }