storage_service: check for CDC flag in start_gossiping
storage_service: check for CDC flag in start_gossiping

This is a bug: we tried to retrieve the CDC streams timestamp even if
CDC flag was not enabled in storage_service::start_gossiping.

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