raft topology: check_and_repair_cdc_streams: don't create a new generation if current one is optimal

[kbr-scylla]

Follow-up to #13683.

In #13683 I left a FIXME in storage_service::raft_check_and_repair_cdc_streams:

// FIXME: check if the current generation is optimal, don't request new one if it isn't

The behavior of the original (gossiper-based) implementation of this API would do exactly that: if the current CDC generation is fine, don't create a new one (hence the "check_and" part of the name).

The new implementation always creates a new generation.

It needs to be modified to do the check first. For that we should refactor the code from generation_service::check_and_repair_cdc_streams (the gossiper-based implementation) - take out the optimality check into a separate function and use it in both places. Note that generation_service::check_and_repair_cdc_streams has other checks which are not applicable to the group0-based implementation (the situation with group 0 is simpler, we don't need to do a distributed read and so on). We basically need only this part:

            if (tmptr->sorted_tokens().size() != gen->entries().size()) {
                // We probably have garbage streams from old generations
                cdc_log.info("Generation size does not match the token ring, regenerating");
                should_regenerate = true;
            } else {
                std::unordered_set<dht::token> gen_ends;
                for (const auto& entry : gen->entries()) {
                    gen_ends.insert(entry.token_range_end);
                }
                for (const auto& metadata_token : tmptr->sorted_tokens()) {
                    if (!gen_ends.contains(metadata_token)) {
                        cdc_log.warn("CDC generation {} missing token {}. Regenerating.", latest, metadata_token);
                        should_regenerate = true;
                        break;
                    }
                }
            }

The test that was modified in #13683 should be modified further to call the API after a decommission multiple times concurrently; only one generation should be created.

[avikivity]

No vulnerable branches, not backporting.