> (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.

{
    
    
    if (shared->merge_tree_disk_selector)
    {
        for (auto & [disk_name, disk] : getDisksMap())
        {
            LOG_INFO(shared->log, "Shutdown disk {}", disk_name);
            disk->shutdown();
        }
    }
    
    auto & tmp_data = shared->temp_data_on_disk;
    if (tmp_data && tmp_data->getVolume())
    {
        auto & disks = tmp_data->getVolume()->getDisks();
        for (auto & disk : disks)
            disk->shutdown();
    }
    shared->shutdown();
}
Context::ApplicationType Context::getApplicationType() const
{
    return shared->application_type;
}
void Context::setApplicationType(ApplicationType type)