LRUCache: Use a shared unload function

[gkjohnson]

Right now every tile gets its own function created but you should be able to set it per cache object:

const cache = new LRUCache();
cache.unloadCallback = tile => {

  // ...

};

[gkjohnson]

It's important for the callback to know which renderer to call "dispose" on so tiles get cleaned up in the right place. A this field could be added so the callback is fired with a certain scope but this seems low priority.

There are also a ton of other callbacks crated in the request contents function used for promises so the benefits of addressing this is unclear.