Fix race condition on status in Relay.Publish method and failure to send

A race-condition exists between setting of the (unprotected) status and the callback which sets the status upon receiving an OK.
The message is sent which can receive an OK in separate goroutine (setting status) prior to the status being set to 'sent.'
The OK can be received prior to the status being set.

This fix also sets the status to PublishStatusFailed if the WriteJSON call fails.

relay.go

@@ -305,10 +305,11 @@ func (r *Relay) Publish(ctx context.Context, event Event) (Status, error) {
 	// publish event
 	message := []any{"EVENT", event}
 	debugLog("{%s} sending %v\n", r.URL, message)
+	status = PublishStatusSent
 	if err := r.Connection.WriteJSON(message); err != nil {
+		status = PublishStatusFailed
 		return status, err
 	}
-	status = PublishStatusSent
 
 	sub := r.PrepareSubscription(ctx)
 	sub.SetLabel("publish-check")