There's a bunch of helpers for CDC gen service in db/system_keyspace.cc. All are static and use global qctx to make queries. Fortunately, both callers -- storage_service and cdc_generation_service -- already have local system_keyspace references and can call the methods via it, thus reducing the global qctx usage.
Closes#11557
* github.com:scylladb/scylladb:
system_keyspace: De-static get_cdc_generation_id()
system_keyspace: De-static cdc_is_rewritten()
system_keyspace: De-static cdc_set_rewritten()
system_keyspace: De-static update_cdc_generation_id()