Make `db_clock` `time_t` conversions `constexpr`
Make `db_clock` `time_t` conversions `constexpr`

duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla