JBAS-1997 Prevent sequential scans in hold queue pos func

JBAS-1997 Prevent sequential scans in hold queue pos func

Brute-force the PG planner to make it use index scans instead of
sequential scans in the hold queue position stored proc.  This resolves
a problem where the planner starts using seq scans over time as the
stats morph in the presence of lots of active, large-queue holds.

Avoidin the seq scan shows consistently better performance.

Signed-off-by: Bill Erickson <berickxx@gmail.com>