Vaadin TreeTable component originally supported partial updates for expanding/collapsing rows. Since Vaadin 7, this mechanism was disabled because it doesn't work with generated components in table cells.

We need to re-enable that and switch to another mechanism for customizing cell content for column formatters (having proper vaadin components in cells on every row is a costly operation anyway).

We take the chance to restore row-based lazy-loading for trees, with a comfortable cache rate so that we're still on the safe side for very big hierarchies.