HPCC-12547 Improve the roxie splitter performance

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-12547 Minor improvements to isLastTailReader()

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6650 from ghalliday/issue12547

HPCC-12547 Improve the roxie splitter performance

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>