"Should I stay or should I go?" This famous question, popularised in the lyrics of a rock tune from the 1980s, has haunted Russian President Vladimir Putin for years.

And for good reason, since Mr Putin, who has already ruled the world's biggest country for over two decades, will be 72 years old by the time his current term of office expires in 2024. And, like all long-serving politicians, he wants to make decisions about his and his nation's future, rather than leave these to biology.