Investigate how much of the vaadin-grid scroller and data-provider can be reused [1 🕒 * 1 👨]

[yuriy-fix]

Child of #657

[tomivirkki]

Most likely makes sense first to..

• extract the <iron-list> fork out from <vaadin-grid> to a separate repo
• with current <vaadin-grid-scroller> extension (bringing "lazy loading" and "million dollar scrolling" features on top of it) and publishing it as an internal component (name TBD)

..and then..

• update <vaadin-grid> to use that (small effort)
• modify <vaadin-combo-box> to use the new scroller element and build the required (clearable) cache layer for lazy loading. Change items API to build on top of the new layer.

[tomivirkki]

Early proto at https://github.com/vaadin/vaadin-combo-box/tree/data-provider-proto

Note: this proto still uses <iron-list> internally. We can postpone the scroller change mentioned above for later. The feature is included as an additional mixin with very little internal changes needed.