> (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.

{
    std::lock_guard lock(shared->clusters_mutex);
    if (!shared->clusters)
    {
        const auto & config = shared->clusters_config ? *shared->clusters_config : getConfigRef();
        shared->clusters = std::make_shared<Clusters>(config, settings, getMacros());
    }
    return shared->clusters;
}
void Context::startClusterDiscovery()
{
    if (!shared->cluster_discovery)
        return;
    shared->cluster_discovery->start();
}
void Context::setClustersConfig(const ConfigurationPtr & config, bool enable_discovery, const String & config_name)