Ignore canceled holds during alt-queue generation

When zero potential copies exist for a hold, the hold queue stats code
determines the size and shape of the queue by finding and sorting holds
that have the same type and target.  This patch forces the code to
ignore canceled holds when looking for similarly-shaped holds.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Jason Etheridge <jason@esilibrary.com>