Battery Subsystem - Make timers not reset with each redeploy

[calvinwt]

    Each time we redeploy code these timers will be reset. This may not true of Timer.getFPGATimestamp(), but that is somewhat unclear. Either way tracking time elapsed across redeploys presents a problem.

Originally posted by @azvanderpas in #234 (comment)

[azvanderpas]

I think we can do this by logging the timers (and other battery stats) every so often with the new DataLogManager and then load the timer values from the last log file.

Think about corner cases here. e.g. what if we replaced the battery?, what if the last deploy crashed?, what if there are no log files yet or the last log file is too old?, etc.