Preserve parent tiles in the source cache

[chrisvoll]

This PR is an alternative to #4210, and aims to resolve an issue where tiles in the source cache are prematurely deleted, resulting in tiles reloading when zooming in and out:

This behavior is visible in this jsbin by clicking the Zoom In button twice, and then the Zoom Out button twice.

Future work

• For raster tiles, updateCacheSize sometimes receives misleading tile size information that results in the cache being too small (e.g., it may think that an 8x8 grid of retina tiles is actually a 4x4 grid). Resolving this should improve the reliability of the cache. I can open a separate issue with more details

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• post benchmark scores
• manually test the debug page

benchmark	master 4995e5d	cv-cache-performance 494d1e2
map-load	126 ms	116 ms
style-load	117 ms	104 ms
buffer	978 ms	984 ms
fps	60 fps	60 fps
frame-duration	3.8 ms, 0% > 16ms	3.8 ms, 0% > 16ms
query-point	0.99 ms	0.96 ms
query-box	69.83 ms	69.10 ms
geojson-setdata-small	10 ms	4 ms
geojson-setdata-large	88 ms	96 ms

cc @lucaswoj — as always, I'm happy to provide any additional info or changes necessary 😄

[lucaswoj]

This is much less scary! Thanks for sticking through and 🚢 this.