High memory consumption with external tilesets

We have a KdTree with ~400.000 (each tile is 200KB) nodes in a single tiles.json (82MB Json file in compact mode). If I open cesium with this tiles.json Chrome crashes and Firefox starts using ~3GB of memory - even if no tile will be loaded. A possible solution for me will be to split tiles.json in multiple once (I hope), but nevertheless someone should take a look at parsing tiles.json to save memory/cpu time (e.g. create Cesium3DTile on demand and not at loading time of tiles.json)

Start by reproducing the failure with the reported 82MB tiles.json in Chrome and Firefox, then trace how tiles.json is parsed and how Cesium3DTile objects are created during loading. Done means the large tileset no longer causes excessive memory use or browser crashes, while preserving on-demand tile loading behavior.

Written by the indexing model from the issue text.