Repeat schedule reminders do not respect changes to original entity

We have a couple of customers experimenting with scheduled membership reminders and they have had a few instances of inappropriate reminders go out. I have narrowed one of the circumstances to being

• if a membership reminder has gone out then scheduled reminders will also go out - regardless of whether the initial event - e.g. membership end data - has changed. I have managed to replicate this in a test

Test & fix for 4.4 here:

https://github.com/civicrm/civicrm-core/pull/4233

BUT NOTE - that I have dealt with the situation where the earliest date a reminder should go out moves (e.g membership_end_date is moved later so don't send out reminders before that date + offset). I have not put any effort into the situation where the date moves earlier. The first possible date is calculated in a very simple way whereas the last date calculation is more complex - so I didn't work through how to reflect that in the reminders. This also reflects the fact I don't have any current situations where this is important.