Bug 7886 - C4/ShelfBrowser slow SQL performance

Bug 7886 - C4/ShelfBrowser slow SQL performance

ShelfBrowser uses two SQL queries with syntax

WHERE ((cn_sort = ? AND itemnumber < ?) OR cn_sort < ?)
AND homebranch = ? AND location = ? AND ccode = ?

homebranch already has index, and adding indexes on ccode and location
improves performance by 30% for each query since MySQL is able to use
index_merge intersect(items_ccode,homebranch,items_location)

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>