Bug 30518: Correct DateTime maths for needs_advancing

Bug 30518: Correct DateTime maths for needs_advancing

The needs_advancing method prior to this patch used basic DateTime
arithmatic, adding a DateTime::Duration in Days to the Arrival date of
the item and then comparing that to today. This, however, can cause bugs
when the arrival + duration date coincides with a DST boundary and as
such may result in an invalid local date.  See
https://metacpan.org/pod/DateTime#Making-Things-Simple for further
details.

This patch updates the code to use the DST safe delta_days method to
count the days between arrival and now instead and then compares this
integer to the defined duration of the stage.

To test:
1. Re-run the unit tests, they should now pass.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>