> (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.

const std::shared_ptr<ASTBackupQuery> & restore_query,
    const OperationID & restore_id,
    const String & backup_name_for_logging,
    const BackupInfo & backup_info,
    RestoreSettings restore_settings,
    std::shared_ptr<IRestoreCoordination> restore_coordination,
    ContextMutablePtr context,
    bool called_async)
{
    std::optional<CurrentThread::QueryScope> query_scope;
    try
    {
        if (called_async)
        {
            query_scope.emplace(context);
            setThreadName("RestoreWorker");
        }
        
        BackupFactory::CreateParams backup_open_params;
        backup_open_params.open_mode = IBackup::OpenMode::READ;
        backup_open_params.context = context;