dist_object<T>::fetch does not correctly handle T with asymmetric serialization

[Amir Kamil]

The spec has dist::object<T>::fetch return a future<T> rather than a future<deserialized_type_t<T>>.

We should also update the rpc equivalent for fetch to return by reference, now that we support that.

[Amir Kamil]

Fix dist_object<T>::fetch for T with asymmetric serialization. Fix implementation of fetch. Fixes #167.

→ <<cset e5da7d9d270b>>