When receiving a signal (INT or TERM) wait for running jobs to
complete instead of terminating them right away.
The new shutdown_timeout configuration setting can be used to force
the termination after a grace delay. If not specified or zero it will
shutdown immediately, for backward compatibility. It will be the case
with existing configuration files or when a configuration file is not
specified.
The config.yml created with the generate-config subcommand will
however default shutdown_timeout to 3h (same as timeout) because it is
likely what a new admin would expect: shutting down waits for jobs to
complete and not abort them.
