HPCC-31160 Fix slow CSV read of super with HEADINGs.

[jakesmith]

A CSV read of a superfile with HEADING() was causing a massive slowdown. Each worker was waiting for the previous to have processed all header lines from all subfiles before moving forward. The net effect was for Thor to process the file sequentially on each worker instead of in parallel.

NB: kept old code that pended on previous worker to have processed all subfile header lines as a failsave expert option.

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Testing:

[github-actions]

https://track.hpccsystems.com/browse/HPCC-31160
Jira updated

[ghalliday]

Looks sensible.