GetTreeState gRPC needs more updates for Orchard

[LarryRuane]

#384 purported to add support for Orchard for GetTreeState, but it looks incomplete. The GetTreeState gRPC handler calls zcashd's z_gettreestate from within a loop. The loop continues, following the z_gettreestate's Sapling.SkipHash until Sapling.Commitments.FinalState is not empty.

What seems wrong is that Orchard's FinalState and SkipHash are not considered in this loop. I don't know what the correct fix is.