By Kane Wu, Scott Murdoch and Donny Kwok HONG KONG, Dec 17 (Reuters) - Cryptocurrency exchange HashKey Holdings Ltd made a flat debut in Hong Kong on Wednesday amid recent weakness in digital assets, but its management ​said it remains confident about the sector's long-term prospects. HashKey shares closed at HK$6.67, down 0.15$6.68. The stock rose as much as 6.6% to HK$7.12 before sliding 8.4$6.12. That compared with a ‌0.9% gain in the benchmark Ha ...