perf: avoid copying entire data *somehow*

[ankush]

Context: #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.

datatable/src/datamanager.js

Lines 213 to 215 in 4f5b242

	row = row
	.map((cell, i) => this.prepareCell(cell, i))
	.map(cell => Object.assign({}, baseRowCell, cell));

[ankush]

Small improvement: #217