Move btbulkdelete's vacuum_delay_point() call to a place in the loop where

we are not holding a buffer content lock; where it was, InterruptHoldoffCount
is positive and so we'd not respond to cancel signals as intended.  Also
add missing vacuum_delay_point() call in btvacuumcleanup.  This should fix
complaint from Evgeny Gridasov about failure to respond to SIGINT/SIGTERM
in a timely fashion (bug #2257).