code: function(tile, tileUrl, done) {
  		return function(err, data) {
  			if (data) {
  				return this._onCacheHit(tile, tileUrl, data, done);
  			} else {
  				return this._onCacheMiss(tile, tileUrl, done);
  			}
  		}.bind(this);
  	}
docs: Returns a callback (closure over tile/key/originalSrc) to be run when the DB backend is finished with a fetch operation.