Add a flag to opt-in to Bucket cleanup.

[jschwartzentruber]

We have Buckets that are very easy to reproduce, that constantly collect crashes that aren't useful after the report has been filed. These buckets grow unbounded because bucketed crashes are not automatically cleaned up.

We should add flag to allow bucket creators to opt-in to the normal periodic crash cleanup threshold. (delete crashes older than x days).

As a safety mechanism, we could add a minimum fill threshold as well (don't cleanup if less than x crashes) so that the bucket is never completely emptied by this cleanup process, if the crash is fixed or stops matching the signature.