Optimize ScalarValue list variant using Array trait object #996

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

This comes out from the arrow2 PR started by @jorgecarleitao. Here is a discussion on why it would be more efficient to model ScalarValue as dyn Array: #68 (comment)

Describe the solution you'd like

Change ScalarValue's List variant to:

List(Option<Arc<dyn Array>>, DataType),

Describe alternatives you've considered

Current way of warping every list value with a ScalarValue enum has a lot of overhead on read/write.

Additional context

We should probably wait for the arrow2 PR merge before starting to work on this.

#68 (comment) has more discussion on how we can handle this in ballista's protobuf ser/de.