By Richard Cowan and Moira Warburton WASHINGTON (Reuters) -Hakeem Jeffries was unanimously elected on Wednesday to become the Democratic Party’s top leader in the U.S. House of Representatives beginning in January, making him the first Black American to hold such a high-ranking position in Congress. The vote by Jeffries’ fellow Democrats also marked the rise of a younger generation of leaders in the 435-member House and the end of the Nancy Pelosi era. In 2007 she became the first woman to be elected House speaker. Jeffries, a 52-year-old New Yorker, will hold the position of House Democratic …