KAFKA-12999: Make RecordHeader reads thread-safe

[mingyen066]

This patch implements
KIP-1205
to address concurrency issues (NPE) related to the lazy initialization
of RecordHeader fields.

The added test verified that concurrent access to RecordHeader.key()
and RecordHeader.value() no longer throws NullPointerException

[chia7712]

@mingyen066 please update the upgrade.html as well

[chia7712]

Or we could leverage CompletableFuture

        CountDownLatch startLatch = new CountDownLatch(1);
        var fs = IntStream.range(0, threads).mapToObj(__ -> CompletableFuture.supplyAsync(() -> {
            try {
                startLatch.await();
                header.key();
                header.value();
                return null;
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        })).collect(Collectors.toUnmodifiableList());
        startLatch.countDown();
        fs.forEach(CompletableFuture::join);

[chia7712]

one more thing: could you please add similar unit test for null value?