Currently, we first print the json contents into a stringstream buffer
and then we write it as a whole to the manifest.json file output stream.
This is not scalable and may cause large allocation for large enough number
of files.
Fixes#24216
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closesscylladb/scylladb#27542