draft expiration warning with zero time left is not useful.

[rjsparks]

cc @russhousley, @bacampbe ,@mskucherawy

Recently draft-ietf-stir-rfc4916-update expired. The chairs, ad, and authors were sent two emails, nearly simultaneously. The second noted that the draft had expired. The first said:

The following draft will expire soon:

Name:     draft-ietf-stir-rfc4916-update
Title:    Connected Identity for STIR
State:    I-D Exists
Expires:  2022-10-23 (in 0 minutes)

That was a waste of resources at best, and a source of confusion at worst.

The expiration warning management command should instead have a configured buffer for which to not warn if the clock is close to running out. I suggest not sending the warning message if the remaining time is less than 12 hours.