Return SnapshotMeta and load exact Dictionary for pivot

Part 2...

This possibly relies a little on @maxpow4h change to `DictionaryId` (I'm happy to wait if that's going to be done soon). 

The more important question is whether we store a commit id in the snapshot meta, or just the dictionary id. Or is there another option?
> Any reason for removing this?

It ended up being the same as `loadMigrate` and the fail case was already being tested.