HPCC-10784 Keyed limits on stepped indexreads can be miscalculated

Don't use the stepped/merging key manager for any pre-counts - it
doesn't work (and would be very inefficient even if it did).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5352 from richardkchapman/keyedlimit-stepping

HPCC-10784 Keyed limits on stepped indexreads can be miscalculated

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>