Make termination conditions more flexible.

[paulgb]

This would introduce a struct to replace grace_period_seconds.

I suggest:

struct ShutdownCondition {
    idle_threshold_seconds: Option<u32>, // replaces grace_period_seconds
    time_limit_seconds: Option<u32>, // total time limit
}

Now that we will have a way to terminate via API (#123), the condition where both values are None is conceivable.