Bug 29134: Use a subquery to increase performance of patron attributes search

Bug 29134: Use a subquery to increase performance of patron attributes search

This patch generates a subquery and checks if a borrowrnumber is in the results to add patrons
to search results

To test:
 1 - Generate a bunch of patrons:
    SELECT surname, firstname, branchcode, categorycode FROM ( SELECT surname FROM borrowers ORDER BY rand() ) a,( SELECT firstname FROM borrowers ORDER BY rand() ) b,( SELECT branchcode FROM borrowers ORDER BY rand() ) c,( SELECT categorycode FROM borrowers ORDER BY rand() ) d LIMIT 50000
 2 - Add a patron attribute to the system and make it searchable - I used code 'TEST'
 3 - Add a value for this attribute to many patrons:
   INSERT INTO borrower_attributes (borrowernumber,code,attribute) SELECT borrowernumber, 'TEST','alphabet' FROM borrowers LIMIT 10000;
 4 - In staff client got 'Patrons'
 5 - Open the browser console (F12) and view the netwrok tab
 6 - Perform a patron search for 'a'
 7 - Note the time it takes for 'search' to complete in console
 8 - Apply patch, restart_all
 9 - Repeat search
10 - Note it is much faster
11 - prove -v t/db_dependent/Utils/Datatables_Members.t

NOTE: I tested with 500k patrons and 100k attributes - search returned in ~2 seconds with patch
and did not return before I got impatient without patch

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>