BOSTON — Dzhokhar Tsarnaev’s attorney is renowned for keeping her high-profile clients off death row, but Judy Clarke could face her most daunting challenge yet in trying to save the Boston Marathon bomber from execution. Most of Clarke’s past successes hinged on persuading prosecutors to take the death penalty off the table before their cases ever reached a jury. She accomplished that on behalf of such killers as Unabomber Ted Kaczynski, Atlanta Olympics bomber Eric Rudolph and Arizona mass …