storage_service: use get_token_metadata and get_mutable_token_metadata methods
storage_service: use get_token_metadata and get_mutable_token_metadata methods

In preparation to converting to using shared_token_metadata internally.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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