HPPC-10010 - Fix race condition in multicore join helper

worker threads creating reader pipe stream too late.
Reader could start reading before created by thread.
Move create to worker constructor.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
HPCC-10010 - set uninitialized stopped flag

Should be inconsequential anyway, but spotted during review

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4818 from jakesmith/hpcc-10010

HPPC-10010 - Fix race condition in multicore join helper

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>