Allow configuring the ttl_hash_set EXPIRE period using the environment

This change allows us to inject the `EXPIRE` time for the crawler rather than setting it as a constant. Recent runs of the crawler are completing within 5 hours so there's no point setting a high value like 48 hours.

Over time, we might want to increase/decrease this and it'll be easier to do with an environment than having to explicitly change some code, built it and then deploy it.

I've also changed the default time from `48h` to `12h`.
I'd prefer to blow up here if we can't parse the user provided value.