Bug 17494: Make sure the same verification token won't be generated twice

Bug 17494: Make sure the same verification token won't be generated twice

Well, this patch does not enforce the unique constraint but mimic what
is already done in C4::ImportExportFramework and Koha::Upload where
md5_hex is used.

Test plan:
Have a look at the code and confirm that it adds more randomness to the
hashed string

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Mason James <mtj@kohaaloha.com>