mutation_partition: Replace hash_row_slice with appending_hash
mutation_partition: Replace hash_row_slice with appending_hash

This enables us to only branch once per row on the actual hash
algorithm, instead of once per row data item.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla