Enhance ByteBufferUtils.findCommonPrefix to compare 8 bytes each time

Currently, the `ByteBufferUtils.findCommonPrefix family of methods compare two buffers a single byte at a time. In reviewing the patch for HBASE-28012, zhangduo suggested that the ByteBufferUtils.findCommonPrefix methods could be enhanced to compare 8 bytes at a time like the ByteBufferUtils.compareToUnsafe family of methods already does (which was added in HBASE-12345)