Add configurable 'Sunset date' for log shards

[mcpherrinm]

While Sunlight is new enough we haven't even gotten logs accepted yet, we'll have to shut down log shards at some point.

It might be useful for us as operators to have a configurable 'sunset date', after which no further certificates are accepted, and no further tree heads are signed.

In typical usage, we might set that to be a few months after the NotAfter limit.

It might be nice to also clean up the log's on-disk cache at that time too, further avoiding requiring any manual intervention or external-to-sunlight automation.

[FiloSottile]

In typical usage, we might set that to be a few months after the NotAfter limit.

Ideally, Sunlight being opinionated and all that, we'd make it automatic a fixed offset after NotAfter. However, why so long after? IMHO one week is enough, and lets operators reclaim space sooner, leading to lower steady-state requirements.

It might be nice to also clean up the log's on-disk cache at that time too, further avoiding requiring any manual intervention or external-to-sunlight automation.

The tuscolo2025h2 cache is just 64G for 1.78T of log, it doesn't feel worth the complexity/risk to clean it up.

[mcpherrinm]

I don't have a really good reason to pick any particular length after the shard is "done", so whether it's a week or a month, anything is fine

The tuscolo2025h2 cache is just 64G

For a particular shard I'm not too worried, but if we're never deleting them, our filesystem is going to fill up in a few years.

My goal with anything like this is to avoid any operation which needs somebody to log into a server and take any action.
It's OK if that's not built into Sunlight; we'll just build it as part of our own automation.