Intervals should reset if the message changes

[flowerysong]

While skimming RFC 3834, the "same reponse" phrasing in this part gave me pause:

   -  Personal and Group responses that are intended to notify the
      sender of a message of the recipient's inability to read or reply
      to the message (e.g., "away from my mail" or "too busy"
      notifications) SHOULD NOT issue the same response to the same
      sender more than once within a period of several days, even though
      that sender may have sent multiple messages.  A 7-day period is
      RECOMMENDED as a default.

Currently simvacation only tracks sender/rcpt for suppression of notifications, but it would be nice if the user updating their message reset the interval so that people who received the old message get the new message sooner.