BOSTON, Nov 9 (Reuters) - Massachusetts' lawmakers on Thursday took a giant step toward killing a proposal to ban gay marriage in the only U.S. state where it is legal. With protesters on both sides of the debate rallying outside the gold-domed statehouse, lawmakers voted 109 to 87 to delay a decision on whether to back a constitutional amendment that would have given voters a chance to ban gay marriage. Gay rights advocates cheered the move, seen as a crushing blow to opponents of gay marriage who had gathered 170,000 signatures in a petition that asked lawmakers to put the...