LIKE ANY good central banker, Jerome Powell is a study in caution when he speaks publicly, choosing each word with the knowledge that even hints of optimism or pessimism can whip up a frenzy in financial markets. But his rhetorical restraint should not be mistaken for a lack of courage or boldness in his piloting of the Federal Reserve through great turbulence. He is now entering the final phase of his chairmanship of the central bank, with one big piece of unfinished monetary business and one big political worry clouding the horizon.