IDataView Cleanup: Rename `UInt128`

[TomFinley]

The row cursor IDs serve a purpose as described here.

https://github.com/dotnet/machinelearning/blob/master/src/Microsoft.ML.Core/Data/ICursor.md#getidgetter

These IDs are represented as 128-bit numbers. Correspondingly, I named them UInt128 back in the long vanished past. However, this is a troubling name: this first of all conflicts with any name .NET would use in the future to represent 128-bit unsigned integers (assuming they ever did so), which is reason enough not to use it. But even without this reason, this type just doesn't quite act like a number.

On the other hand, I am not certain what a good name would be. Something like RowId, RecordId, or something like that?

/cc @Zruty0 @shauheen @terrajobst

[Zruty0]

I like RowId better. Or even UniqueId.

[TomFinley]

I like RowId. UniqueId is bad since they are only probabilistically unique, and people might get confused between that and GUIDs (which go further and are globally unique), which this definitely is not.