WIP: wrap stored arrays in dask.delayed

[shoyer]

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[jhamman]

@shoyer - this seems to have gone stale. Is this still worth pursuing?

[shoyer]

I think we should drop this, unless we can find an example where it actually make a performance difference after the backends refactor.