HPCC-12441 Unordered join deadlock if stopped early

The way the unordered join helper is organised (a bit weirdly),
meant that the reader continued pumping work for the workers
and the underlying joinhelper needed stopping before everything
else could stop.

This commit fixes that and rationalizes the join helper abort a
bit.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>