Allow large features to span multiple reducers on ingest

I hope this is OK. A few points:

1. It's actually probably work creating a new thrift lookup class for `offset` and `count` and share that now with squash, or should I leave the `int` stuff for now?
2. Not happy with the `SkewSpec`. I've added an arbitrary, but I know I'm missing a better property.
3. The `RecreateSpec` broke because it was picking up the `.version` factset file and calculating `x / 0` because there are no features.