Auto refreshing cache

[wilhelmberg]

fix #261

Updates ambient caching to not only request tiles when the cache has reached its maximum size but also automatically check in the background if a newer version of already locally cached tiles is available.

• If a tile exists in the local cache it is immediately returned.
• Afterwards a HEAD request is sent to the API to check for a newer version of that tile.
• If yes, then the new tile gets downloaded and put into the local cache
• Then the (new) tile is returned to the calling code a second time

Also adds a new item to the Mapbox menu to delete all existing cache files not just the one of the current scene.

TODO:

• expose settings in Mapbox -> Setup to toggle that behaviour with default:OFF
• keep MapboxAccess.ClearCache() for backwards compatibility but mark it as deprecated
• clean up code/remove log statements
• delete test scene

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As part of the process of submitting this PR, please:

• Document the PR changes
• Update the changelog

[brnkhy]

@BergWerkGIS I tried the steps you posted on slack and it worked as you said. So I saw tile appear, disappear and reload with new data. I couldn't test it any other way though, so let me know if you want me to try anything else.

[brnkhy]

@BergWerkGIS
So I tested this again, especially the destroy part.
Pushed a small change, with just three lines. Instead of destroying the child objects, I'm unregistering tiles which lead into all those objects getting pooled and reused.
Did a few performance tests, surprisingly destroying objects looks much faster than I though. Especially when there isn't too many of them. Creating was kinda same, faster than I thought. So there isn't a huge speed different between them, a few tests just shown caching is maybe little faster.
Bigger thing is memory of course. Destroying objects leaks memory as Unity doesn't destroy mesh objects when you do that. So behind the scenes, when you destroy and recreate tiles, you end up with 2x mesh memory. Unregistering tiles doesn't have this issue of course.
One thing I couldn't test good was the correctness though. There's a custom tileset and you know it better so I thought maybe you can quickly test if that unregistering thing is also correct.