Reduce the EXPIRE period to 12 hours for crawled URLS

Having tested the crawler a few times through it completes well within
4-6 hours. Given this time period, we should reduce the default for
the EXPIRE period to something more sensible that allows us to crawl
pages more frequently.

Later on, this should be set using a ENV variable that's passed in.
Add a Ping() method to TTLHashSet() for health checking

When we want to health check the underlying redis service we can use
this helper method on the object.