Optimize http requests - threaded tile fetching

[wilhelmberg]

refs #11

Tile should be fetched in a parallel manner.

Associated tickets: https://github.com/mapbox/mapbox-sdk-unity-core/issues?utf8=%E2%9C%93&q=is%3Aissue%20%22Optimize%20http%20requests%22

[wilhelmberg]

Dang!
Just realized during splitting the multithreading work into core and sdk related items, that I'm using the UnityToolbag.Dispatcher in Map.cs:
core isn't a standalone solution anymore.

So, some more work is needed ☹️ to check in core if we are being called from Unity or not and adjust the threading strategy accordingly.

[wilhelmberg]

Implemented our own Mapbox.Threading.Dispatcher based in UnityToolBag.Dispatcher in #40