> (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.

try
{
    LOG_DEBUG(log, "Setting up {} to store temporary data in it", path);
    fs::create_directories(path);
    
    fs::directory_iterator dir_end;
    for (fs::directory_iterator it(path); it != dir_end; ++it)
    {
        if (it->is_regular_file())
        {
            if (startsWith(it->path().filename(), "tmp"))
            {
                LOG_DEBUG(log, "Removing old temporary file {}", it->path().string());
                fs::remove(it->path());
            }
            else
                LOG_DEBUG(log, "Found unknown file in temporary path {}", it->path().string());
        }
        
    }
}
catch (...)
{
    DB::tryLogCurrentException(log, fmt::format(
        "Caught exception while setup temporary path: {}. "
        "It is ok to skip this exception as cleaning old temporary files is not necessary", path));
}
static VolumePtr createLocalSingleDiskVolume(const std::string & path)