HPCC-10528 New threaded concat code can lock up

Improper handling of readyPending (the optimization to just use them up rather
than signalling all at once is not valid, as they would only get used up when
!inGroup).

Also a potential race condition in the eof handling in peek.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5203 from richardkchapman/concat-lockup

HPCC-10528 New threaded concat code can lock up

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>