Motivations:
1. There are duplicated implementations of decoding hex strings. #6797
2. ByteBufUtil.HexUtil.decodeHexDump does not handle substring start
index properly and does not decode hex byte rigorously.
Modifications:
1. Function decodeHexByte is moved from QueryStringDecoder into ByteBufUtil.
2. ByteBufUtil.HexUtil.decodeHexDump is changed to use decodeHexByte.
3. Tests are Updated accordingly.
Result:
Fixed#6797 and made hex decoding functions more robust.
629b83e0a5