getCacheRecord(cacheKey) {
            $.console.assert(cacheKey, '[TileCache.getCacheRecord] cacheKey is required');
            return this._cachesLoaded[cacheKey] || this._zombiesLoaded[cacheKey];
        }
Get cache record (might be a unattached record, i.e. a zombie)
@param cacheKey
@returns {OpenSeadragon.CacheRecord|undefined}