Alternative destination-specific serialization

In some cases objects may want to serialize themselves dependent on the sender and recipient machine. One such example is multi-gpu settings where certain workers may be able to exchange GPU data efficiently if they are on the same physical host or nearby in the network.

To meet this goal it may make sense to enable host-specific serialization, that is we would optionally use a new serialization function that was invoked for certain types:

```
serialize(obj, host, destination) -> bytes
```

We would probably have to register serialization functions for certain types or else look for a special protocol like `__getstate_host__` and `__setstate_host__`.

cc @seibert @sklam @gforsyth