Two edge cases were previously missed:

1. Infinite should stay infinite. Prefixes with infinite lifetimes
   should be advertised as such. Likewise, should prefixes with
   infinite lifetimes be added to the prefix list as such
2. Prefixes with lifetimes > (UINT32_MAX - 1) / 1000 should have the
   longest possible lifetime below infinite (UINT32_MAX - 1)

This fixes that.