[BUG] Consecutive PDO writes are handled incorrectly

If I'm setting two values in a PDO, e.g.

```
device.setValue("Frame tv_sec", sec);
device.setValue("Frame tv_nsec", nsec);
```

the observed behavior is that if both values are updated, a change in the first value triggers its own pdo message and then a change in the subsequent value causes a second pdo message to be sent instead of sending both at the same time in a single message.