rgw_lc: replace strftime w/fmt and chrono:calendar

rgw_lc:  replace strftime w/fmt and chrono:calendar

It's reliably claimed that std::strftime is not
mt-safe, and this would be a likely root cause of
intermittent scrambled expiration header output cases
that have been reported.

Fixes: https://tracker.ceph.com/issues/63973
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>