HPCC-11937 Fix race condition in smart join
HPCC-11953 Throw error when RIGHT rows would be lost

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6267 from ghalliday/issue11937_4

HPCC-11937 Fix race condition in smart join

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>