> (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 (ConfigHelper::getBool(*config, "allow_experimental_cluster_discovery") && enable_discovery && !shared->cluster_discovery)
    {
        shared->cluster_discovery = std::make_unique<ClusterDiscovery>(*config, getGlobalContext());
    }
    
    if (shared->clusters && isSameConfiguration(*config, *shared->clusters_config, config_name))
        return;
    auto old_clusters_config = shared->clusters_config;
    shared->clusters_config = config;
    if (!shared->clusters)
        shared->clusters = std::make_shared<Clusters>(*shared->clusters_config, settings, getMacros(), config_name);
    else
        shared->clusters->updateClusters(*shared->clusters_config, settings, config_name, old_clusters_config);
}
void Context::setCluster(const String & cluster_name, const std::shared_ptr<Cluster> & cluster)
{
    std::lock_guard lock(shared->clusters_mutex);
    if (!shared->clusters)
        throw Exception("Clusters are not set", ErrorCodes::LOGICAL_ERROR);
    shared->clusters->setCluster(cluster_name, cluster);
}
void Context::initializeSystemLogs()
{
    auto lock = getLock();
    shared->system_logs = std::make_unique<SystemLogs>(getGlobalContext(), getConfigRef());
}
void Context::initializeTraceCollector()