I have not had many relaxing massages in my life – they tend to leave me more sore than satisfied – but the one I was given at a 24-hour spa in Shenzhen, in China’s Guangdong province, just a stone’s throw from Hong Kong, lulled me into a Zen-like state so profound that I became entirely detached from this world.The internet’s obsession with 24-hour spas is nothing new – jjimjilbangs, or Korean bathhouses, have long been essential stopovers for visitors to South Korea – but the focus on China’s mega-spas is a more recent trend. It is easy to see…