mutation_partition: use proper hasher in row hashing
mutation_partition: use proper hasher in row hashing

Instead of using the default hasher, hasing specializations should
use the hasher type they were specialized for. It's not a correctness
issue now because the default hasher (xx_hasher) is compatible with
its predecessor (legacy_xx_hasher_without_null_digest), but it's better
to be future-proof and use the correct type in case we ever change the
default hasher in a backward-incompatible way.

Message-Id: <2fd251174d82d176fa403b0df0f35bba6f308a29@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla