Use a skin cache system + store Minecraft accounts with their UUID

Use the skin-cache library to cache the skins and serve them locally. Caching not really needed but the library offers flexibility in changing sources/schemes e.t.c.

When an account logs in to the auth server their 'account' is created UUID=>username. If someone with a different UUID logs in with the same username the old account username will then be 'null' and show as 'Unknown Account'.