Make the `EXPIRE` time for ttl_hash_set configurable through the ENV

Instead of making the expiry time a constant, allow it to be injected
using configuration. This can then be later owned by the environment
that's passed in to be more in keeping with the 12-factor style apps
we try to write.
Inject the `EXPIRE` time for ttl_hash_set using an ENV variable

Read the expire time from the environment variable that we define and
pass it through the call to `NewTTLHashSet`.
Blow up with a fatal error if we can't parse `TTL_EXPIRE_TIME`
Be consistent and use `time.Hour` for test `EXPIRE` time
Merge pull request #67 from alphagov/allow-configuring-the-ttl-hash-set-ttl-period

Allow configuring the ttl_hash_set EXPIRE period using the environment