perf: avoid copying entire data *somehow*

[ankush]

Context: https://github.com/frappe/datatable/pull/197

Datatable's list virtualization works for the most part but this code is a limiting factor in loading large data. Because ALL data gets copied here, it's not possible to optimize this further without a design change that does lazy materialization.

https://github.com/frappe/datatable/blob/4f5b2422ec893bbfc98b75d2a3037ae3a3e0b204/src/datamanager.js#L213-L215

[ankush]

Small improvement: https://github.com/frappe/datatable/pull/217