Bug 21562: Fix sort by checkout date in the circulation table

Bug 21562: Fix sort by checkout date in the circulation table

The sort is done in SQL but resort by DataTables when displaying the
results.
iDataSort should be used to mimick date_due sort behavior.

Test plan:
- Check some items out
- Modify the checkout and due dates, like:
    2018-10-22
    2018-01-23
    2018-05-27
- Sort by checkout date, then due date. The sort should be correct for
all values of the DateFormat syspref

Followed test plan, patch works as described and passes QA test tool

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>