partition_version: Add const-quialified variant of operator->
partition_version: Add const-quialified variant of operator->

[tgrabiec: Extracted from a different patch]

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