CAMBRIDGE - In the world of economics and finance, revolutions occur rarely and are often detected only in hindsight. But what happened on February 19 can safely be called the end of an era in global finance. On that day, the International Monetary Fund published a policy note that reversed its long-held position on capital controls. Taxes and other restrictions on capital inflows, the IMF's economists wrote, can be helpful, and they constitute a legitimate part of policymakers' toolkit.