Bug 12371 - Links in every patron self-registration email points to a single borrower

If multiple registrations are submitted, the first patron to register
will be used for the first patron to click the registration confirmation
link!

Test Plan:
1) Submit 2 new patron registrations
2) Use the confirm link from the 2nd registration
3) Note you end up registering as the first submitted registration
4) Apply the patch
5) Repeat steps 1 and 2
6) Note you are now confirmed correctly

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>

Test plan appears to work fine, I have a feeling the sql could be
written better but can't come up with it on a Sunday morning

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described and fixes a critical bug.
Passes tests and QA script.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>