By Frank Dalleres The last time Savannah Marshall and Claressa Shields met in a boxing ring, they were both amateurs with no realistic prospect of making a good living from the sport. A decade on, the bitter rivals are due to renew hostilities at the top of the professional ranks on a historic occasion that promises to earn them $1m each. The purse illustrates the profile of Saturday’s middleweight unification fight between England’s Marshall and American Shields which, it is hoped, will sell out London’s 20,000-capacity O2. It is the headline act on the first elite all-female card ever seen i…