Currently, when computing the mutation to be stored in system.batchlog,
we go through data_value. In turn this goes through `bytes` type
(#24810), so it causes a large contiguous allocation if the batch is
large.
Fix by going through the more primitive, but less contiguous,
atomic_cell API.
Fixes#24809.
Closesscylladb/scylladb#24811
(cherry picked from commit 60f407bff4)
Closesscylladb/scylladb#24845