[core] pre-cache map tiles

[onlyjsmith]

Paraphrased from a TODO in the source code:

At boot, can we start getting the tile-cover thing going, to cache all needed vector tiles? https://github.com/mapbox/tile-cover

[TNgidi]

Comment by @onlyjsmith on duplicate ticket #138

• SQL.js in WebWorker https://hackernoon.com/execute-millions-of-sql-statements-in-milliseconds-in-the-browser-with-webassembly-and-web-workers-3e0b25c3f1a6
• https://github.com/systemed/tilemaker
• https://openmaptiles.com/extracts/
• https://github.com/NYCPlanning/geojson2mvt