> (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);
     StoragePolicyPtr tmp_policy = getStoragePolicySelector(lock)->get(policy_name);
    if (tmp_policy->getVolumes().size() != 1)
            throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG,
            "Policy '{}' is used temporary files, such policy should have exactly one volume", policy_name);
    VolumePtr volume = tmp_policy->getVolume(0);
    if (volume->getDisks().empty())
         throw Exception("No disks volume for temporary files", ErrorCodes::NO_ELEMENTS_IN_CONFIG);
    for (const auto & disk : volume->getDisks())
    {
        if (!disk)
            throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG, "Temporary disk is null");
        
        DiskPtr disk_ptr = disk;
        if (dynamic_cast<const DiskLocal *>(disk_ptr.get()) == nullptr)
        {
            const auto * disk_raw_ptr = disk_ptr.get();
            throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG,
                "Disk '{}' ({}) is not local and can't be used for temporary files",
                disk_ptr->getName(), typeid(*disk_raw_ptr).name());
        }
        setupTmpPath(shared->log, disk->getPath());
    }
    shared->temp_data_on_disk = std::make_shared<TemporaryDataOnDiskScope>(volume, max_size);
}
void Context::setTemporaryStorageInCache(const String & cache_disk_name, size_t max_size)