ISSUE-2064: DirectMemoryCRC32Digest.update(ByteBuf) allocates lots of byte[]

[sijie]

Original Issue: apache#2064

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FEATURE REQUEST

1. Please describe the feature you are requesting.
   Profiling shows the code block in bold is hit every time an entry is read from bookie.

    @Override
    public void update(ByteBuf buf) {
        int index = buf.readerIndex();
        int length = buf.readableBytes();

        try {
            if (buf.hasMemoryAddress()) {
                // Calculate CRC directly from the direct memory pointer
                crcValue = (int) updateByteBuffer.invoke(null, crcValue, buf.memoryAddress(), index, length);
            } else if (buf.hasArray()) {
                // Use the internal method to update from array based
                crcValue = (int) updateBytes.invoke(null, crcValue, buf.array(), buf.arrayOffset() + index, length);
            } else {
                **// Fallback to data copy if buffer is not contiguous
                byte[] b = new byte[length];
                buf.getBytes(index, b, 0, length);
                crcValue = (int) updateBytes.invoke(null, crcValue, b, 0, b.length);**
            }
        } catch (IllegalAccessException | InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    }

This is due to ByteBuf being marked as read only (as a result buf.hasArray() == false) from

ReadCompletion
public void handleV3Response(BookkeeperProtocol.Response response) {
            readEntryOutstanding.dec();
            ReadResponse readResponse = response.getReadResponse();
            StatusCode status = response.getStatus() == StatusCode.EOK
                ? readResponse.getStatus() : response.getStatus();
            ByteBuf buffer = Unpooled.EMPTY_BUFFER;
            if (readResponse.hasBody()) {
                **buffer = Unpooled.wrappedBuffer(readResponse.getBody().asReadOnlyByteBuffer());**
            }

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have).
   Are you currently using any workarounds to address this issue?

This generates lots of garbage. See attached image. Would be nice to get your thoughts on this issue, maybe using a buffer pool.

3. Provide any additional detail on your proposed use case for this feature.