By Daniel Leussink and Akiko Okamoto TOKYO (Reuters) – When Japan handed Tokyo bus driver Keiki Nambu and his wife, Takako, $870 for each of their nine children, they spent it exactly as the government had feared: paying down a mortgage instead of going shopping. That kind of financial prudence has helped Japanese households amass a staggering $17 trillion in assets over the years, with more than half of that parked in savings. But it also represents a headache for policymakers, who struggle to kick-start consumption and boost a moribund economy. Prime Minister Fumio Kishida’s government has p…