In parallelized aggregation functions super-coordinator (node performing final merging step) receives and merges each partial result in parallel coroutines (`parallel_for_each`).
Usually responses are spread over time and actual merging is atomic.
However sometimes partial results are received at the similar time and if an aggregate function (e.g. lua script) yields, two coroutines can try to overwrite the same accumulator one after another,
which leads to losing some of the results.
To prevent this, in this patch each coroutine stores merging results in its own context and overwrites accumulator atomically, only after it was fully merged.
Comparing to the previous implementation order of operands in merging function is swapped, but the order of aggregation is not guaranteed anyway.
Fixes#20662Closesscylladb/scylladb#24106
(cherry picked from commit 5969809607)
Closesscylladb/scylladb#24388
712d783164