> (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->storage_policies_mutex);
    if (shared->merge_tree_disk_selector)
        shared->merge_tree_disk_selector
            = shared->merge_tree_disk_selector->updateFromConfig(config, "storage_configuration.disks", shared_from_this());
    if (shared->merge_tree_storage_policy_selector)
    {
        try
        {
            shared->merge_tree_storage_policy_selector = shared->merge_tree_storage_policy_selector->updateFromConfig(
                config, "storage_configuration.policies", shared->merge_tree_disk_selector);
        }
        catch (Exception & e)
        {
            LOG_ERROR(
                shared->log, "An error has occurred while reloading storage policies, storage policies were not applied: {}", e.message());
        }
    }
    if (shared->storage_s3_settings)
    {
        shared->storage_s3_settings->loadFromConfig("s3", config, getSettingsRef());
    }
}
const MergeTreeSettings & Context::getMergeTreeSettings() const