LP1864056: Don't compare multiple new Date()s

LP1864056: Don't compare multiple new Date()s

This branch addresses a race condition when
checking out or renewing items.
(The use of minDate in items_out was implicitly
immune to this issue, but that is made explicit
with this patch.)

To test
-------
[1] The race condition would be difficult to reliably
    reproduce directly, although a tool like
    https://github.com/mattzeunert/javascript-clock-speedup
    might help simulate the problem, so to test, verify
    that the following actions do not break:

    * checking out a loan, both with and without
      setting a specific due date
    * renewing a loan with a specific due date
      from the patron items out tab
    * renewing a loan from Circulation -> Renew Items,
      both with and without setting a specific due date.

Signed-off-by: Jason Boyer <JBoyer@eoli.info>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>