By Colleen Goko JOHANNESBURG, Feb 18 (Reuters) - Africa's biggest economies Nigeria and South Africa are driving the strongest growth in demand for stablecoins and are the most optimistic about their potential, ‌a survey found, with many users also wanting them to become more widely accepted. Stablecoins are promising ‌faster and cheaper ways to move money in poorer countries, but as 99% of them, including the dominant Tether and USDC, are ​pegged to the dollar, they also raise worries about economic dol ...