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>
In fact it would be interesting to search the whole source for such comments to see what still lurks around in the dark. Maybe even using something other than //MORE: to indicate a possible bug rather than it's use as "this is not perfect , this should be done instead perhaps but not now"?