Timestamp naming conventions: totp_timestamp vs last_otp_at

[rmm5t]

I'd like to suggest a more consistent naming convention to replace the new totp_timestamp column in preparation for v2.0. A better name would be something like last_otp_at. This would better match Rails naming conventions, it would better coincide with direct_otp_sent_at, and this would match rotp's recommendation.

I'm happy to provide a pull-request for this, but I'd like to get some sense that it's likely to be accepted first and/or if it interests others that use this library.