Function messaging_service::get_rpc_client() suppose to either return existing client or create one and return it. The function is suppose to be atomic, so after checking that requested client does not exist it is safe to assume emplace() will succeed. But we saw bugs that made the function to not be atomic. Lets add an assert that will help to catch such bugs easier if they will happen in the future. Message-Id: <20190326115741.GX26144@scylladb.com>