Shadow runs with a default log level of "info", which can lead to a large number of log messages. We should consider increasing the default log level so that we print fewer log messages by default.

The user can use the general.heartbeat_info option to have Shadow print periodic messages to show that Shadow is still running and not stuck. The general heartbeat messages currently log usage stats, but if we implement #1399, we can reuse these general heartbeat messages to log the simulation progress (current-time, current-simulation-time, % (based on current-sim-time/stop-time)).