HPCC-29923 Avoid CRowServer cancel if not running

[jakesmith]

This avoids an inherent delay being introduced by the MP layer.

CRowServer is used by Global Group, Global TopN and a refactoring
form of IndexWrite (where is merges down to less parts).

Type of change:

• This change is a bug fix (non-breaking change which fixes an issue).
• This change is a new feature (non-breaking change which adds functionality).
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• This change fixes warnings (the fix does not alter the functionality or the generated code)
• This change is a breaking change (fix or feature that will cause existing behavior to change).
• This change alters the query API (existing queries will have to be recompiled)

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• My code follows the code style of this project.
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Testing:

[github-actions]

https://track.hpccsystems.com/browse/HPCC-29923
Jira updated

[ghalliday]

@jakesmith One comment - looks good.
Is 9.2.x the correct target?

[ghalliday]

trivial: not needed (it will be set to false by the compare_exchange

[jakesmith]

removed.

[jakesmith]

Is 9.2.x the correct target?

I figured that it's probably safe, but a very long standing issue that doesn't need to go into 9.0.x, so 9.2.x was more appropriate,
but happy to change it to 9.0.x if you think it would be good to?

[ghalliday]

@jakesmith does it only affect groups? It is a bug fix which is relatively trivial so I think 9.0.x would be appropriate.

@GordonSmith if Jake agrees with my comment and removes the assignment then it can be merged.

[jakesmith]

@jakesmith does it only affect groups? It is a bug fix which is relatively trivial so I think 9.0.x would be appropriate.

I've updated the commit/PR message, but as per the JIRA title, it is also used in Global TopN and a unusual indexwrite case.

@GordonSmith if Jake agrees with my comment and removes the assignment then it can be merged.

unnecessary assignment removed.
Rebased to 9.0.x

[jakesmith]

@GordonSmith if Jake agrees with my comment and removes the assignment then it can be merged.

@GordonSmith - rebased to 9.0.x - I think you can merge it.