Fix memory cache

[gkjohnson]

Fix #761

[gkjohnson]

cc @AnthonyGlt - this should be ready if you want to test it a bit. I just have to write some more tests / fix the existing ones.

In simpler terms, the changes are:

• Split the "max size" eviction and the "min size" eviction while loops so the logic is easier to follow.
• Add a concept of "fully loaded" items. If an item isn't full loaded then it can be evicted from the cache if it's above the "max" size even if its marked as "used". This lets us stop any downloading or yet-to-be-parsed tile geometry if the cache has suddenly become full.
• The loading logic now checks the if the geometry memory usage has been assigned before determining if tile data should be evicted after loading.

[gkjohnson]

PR should be ready, now.

[AnthonyGlt]

nit There is a word missing here

[AnthonyGlt]

the test check is a good addition 👍

[gkjohnson]

Thanks - just reworded it. Let me know once you think everything looks good and we can get this merged and a new version published.

[AnthonyGlt]

Looks good to me 👍
We got to juggle with the notions of used and loaded but at the end, it makes the cache management more subtle.
Well done :D

[gkjohnson]

Thanks - yeah nomenclature could probably use some work but I think the functionality is good.