Suboptimal terrain cache invalidation

Before pruning, entries should be order by most recent access not by LastWriteTimeUtc.

The cache for my main map was invalidated after starting just one new test game:

1. main map created
2. test map1
3. test map2
4. test map3
5. test map4
6. main map (multiple times, still valid but about to get pruned)
7. test map5
8. main map [over 5 entries, cache invalidated because main map is the "oldest"]