> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

{
    while (true)
    {
        ConfigurationPtr cluster_config;
        {
            std::lock_guard lock(shared->clusters_mutex);
            cluster_config = shared->clusters_config;
        }
        const auto & config = cluster_config ? *cluster_config : getConfigRef();
        auto new_clusters = std::make_shared<Clusters>(config, settings, getMacros());
        {
            std::lock_guard lock(shared->clusters_mutex);
            if (shared->clusters_config.get() == cluster_config.get())
            {
                shared->clusters = std::move(new_clusters);
                return;
            }
            
        }
    }
}
std::shared_ptr<Clusters> Context::getClusters() const