> (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.

{
    auto disk = std::make_shared<DiskLocal>("_tmp_default", path, 0);
    VolumePtr volume = std::make_shared<SingleDiskVolume>("_tmp_default", disk, 0);
    return volume;
}
void Context::setTemporaryStoragePath(const String & path, size_t max_size)
{
    shared->tmp_path = path;
    if (!shared->tmp_path.ends_with('/'))
        shared->tmp_path += '/';
    VolumePtr volume = createLocalSingleDiskVolume(shared->tmp_path);
    for (const auto & disk : volume->getDisks())
    {
        setupTmpPath(shared->log, disk->getPath());
    }
    shared->temp_data_on_disk = std::make_shared<TemporaryDataOnDiskScope>(volume, max_size);
}
void Context::setTemporaryStoragePolicy(const String & policy_name, size_t max_size)