Since `_partition_size_entry` and `_rows_in_partition_entry`
are accessed at the same time when updated, and similarly
`_cell_size_entry` and `_elements_in_collection_entry`,
place the member pairs closely together to improve data
cache locality.
Follow the same order when preparing the
`scylla_metadata::large_data_stats` map.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>