Italy’s Banca Monte dei Paschi di Siena SpA is the world’s oldest bank. It’s also one of the most problematic after receiving more than 9 billion euros ($10.6 billion) in bailouts since the global financial crisis from the Italian government, which now has until the end of 2021 to exit its 64% stake. A possible sale to UniCredit SpA is raising more questions about the ballooning taxpayer cost of keeping the bank alive.