Owner API should leverage VersionedSlate and not Slate directly

[antiochp]

Currently the Owner API uses Slate directly. This is too closely coupled to the internals of grin core. We should break this coupling and leverage VersionedSlate.

Are there any reasons not to do this or why this would cause problems?

[yeastplume]

I don't know of a reason to not do this, am happy enough to make the change and test.

Can we hold off on this and #129 for a couple of days? The changes within #200 are a bit more pervasive than I thought originally, and I'd just like to get them in first before dealing with (and particularly testing) slate changes.