HPCC-12471 Fix pull distribute regression

HPCC-12409 introduced a regression whereby the merge stream
could get out of order, because >1 write handler causes the row
packets to arrive in indeterminate order.
Restrict pull distriubute to 1 write handler per target.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6580 from jakesmith/hpcc-12471

HPCC-12471 Fix pull distribute regression

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>