@Override
    Long deserializeCacheKey(final Bytes cacheKey) {
        final byte[] binaryKey = bytesFromCacheKey(cacheKey);
        return timestampExtractor.apply(binaryKey);
    }
Merges two iterators. Assumes each of them is sorted by key
streams/src/main/java/org/apache/kafka/streams/state/internals/MergedSortedCacheWindowStoreIterator.java