Fix a bug that causes the daemon to freeze when shut down with a signal like
SIGINT or SIGTERM, and not with `mina client stop`. The frontier visualization
debug diagram that is written at shutdown will now display a total_currency of 0
for every mask by default. The environment variable
`MINA_FRONTIER_CURRENCY_VISUALIZATION` can be set to re-enable the visualization
if desired; it should fast for smaller testnets.
