MultiBox now handles sync-async

Will be required if when we do SNMP, because that one should be async.
Seems like a job for `filter`...

```python
# lists
sync = filter(lambda item: isinstance(item, Async), values)
async = filter(lambda item: isinstance(item, Sync), values)
dont_care = filter(lambda item: item not in sync and item not in async, values)
# or a little shorter
dont_care = filter(lambda item: not (item in sync or item in async), values)
# or the crappy way
dont_care = [item for item in values if item not in async and item not in sync]
# or, if items are hashable and order doesn't matter (I'm just sorta making this one up)
dont_care = set(values) - (set(sync) | set(async))
```