HPCC-9253 Significant semaphore overhead in the unordered concat

First stab at an improved version (2-3x faster on the attached tests)

Switch to using round-robin tracking to enforce grouping, meaning we can
consider (in a later commit) using fixed size buffers for all inputs.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5129 from richardkchapman/ordered-concat

HPCC-9253 Significant semaphore overhead in the unordered concat

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>