HPCC-8895 DICTIONARY should keep first row with a given key

For consistency with dedup, lookup join, and others, it would make a lot more
sense to keep the first matching record when multiple records with the same
key are added to a dictionary (rather than the last as at present).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4036 from richardkchapman/dict-keepfirst

HPCC-8895 DICTIONARY should keep first row with a given key

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>