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Alternator Streams' "GetRecords" operation has a "Limit" parameter on how many records to return. The DynamoDB documentations says that the upper limit on this Limit parameter is 1000 - but Alternator didn't enforce this. In this patch we begin enforcing this highest Limit, and also add a test for verifying this enforcement. As usual, the new test passes on DynamoDB, and after this patch - also on Alternator. The reason why it's useful to have *some* upper limit on Limit is that the existing executor::get_records() implementation does not really have preemption points in all the necessary places. In particular, we have a loop on all returned records without preemption points. We also store the returned records in a RapidJson vector, which requires a contiguous allocation. Even before this patch, GetRecords had a hard limit of 1 MB of results. But still, in some cases 1 MB of results may be a lot of results, and we can see stalls in the aforementioned places being O(number of results). Fixes #23534 Signed-off-by: Nadav Har'El <nyh@scylladb.com> Closes scylladb/scylladb#23547