HPCC-30690 Nested stats from workers not accumulated correctly

[shamser]

Nested stats from worker nodes were being merged rather than set, meaning that the updates of the nested stats from the worker were added to the stats recorded previously. Graph master should 'set' the stats from workers rather than 'merge'.

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• This change alters the query API (existing queries will have to be recompiled)

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Testing:

[ghalliday]

Please can you target this to 9.4.x though - since it is a bug fix.