Back in 2017 (5a2439e702), we introduced a check for large
allocations as they can stall the memory allocator. The warning
threshold was set at 1 MB. Since then many fixes for large allocations
went in and it is now time to reduce the threshold further.
We reduce it here to 128 kB, the natural allocation size for the
system. A quick run showed no warnings.
Closesscylladb/scylladb#23975