BOSTON — After Boston Marathon bomber Dzhokhar Tsarnaev was convicted in 2015, the family of the youngest victim urged prosecutors to abandon their bid for the death penalty, warning that years of appeals would only keep him in the spotlight and prolong their unthinkable suffering. Five years later, the prospect of a new trial to decide whether Tsarnaev should be executed after an appeals court tossed the 27-year-old’s death sentence has brought anger and anguish to a community in many ways still healing from the April 15, 2013, attack.