IS_UTC_DATETIME should not (always) enforce minimum on historic records

[nursix]

In most use-cases it is desirable to be able keep date(time) values when editing historic records.

Those date(time)s would have met the IS_UTC_DATETIME minimum-requirement at the time of entry, but do no longer as time has progressed.

Therefore, the validator should enforce the minimum-requirement only for new entries (unless explicitly configured otherwise).

To achieve that, the validator should have a set_self_id() method. If that method is called with a record ID (which happens during record updates), then the validator instance should suppress subsequent minimum checks.