Also note that you should NOT roll ntime for data retrieved without explicitly
checking that it falls within the template's limitations (mintime, maxtime,
mintimeoff, and maxtimeoff); read the BIP 23 specification in detail to
understand how they work. It is usually best to simply get more data as often
as it is needed. For blkmk_get_mdata, you may specify that you intend to roll
the ntime header exactly once per second past usetime - it will then set
*out_expires such that the expiration occurs before you roll beyond any ntime
limits. If you are rolling ntime at any rate other than once per second, you
should NOT specify can_roll_ntime to blkmk_get_mdata, and must check that your
usage falls within the explicit template limits yourself.