By Alasdair Pal and Uditha Jayasinghe COLOMBO (Reuters) -Sri Lanka’s prime minister resigned on Monday to make way for a unity government that would try to find a way out of the country’s worst economic crisis in history, but protesters said they also wanted his brother to stand down as president. Prime Minister Mahinda Rajapaksa’s resignation came hours after clashes broke out in Colombo, where supporters of the ruling party stormed an anti-government protest camp and were beaten back by police using tear gas and water cannon. Ruling party parliamentarian Amarakeerthi Athukorala died after a …