> (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.

{
        return (status == BackupStatus::BACKUP_CREATED) || (status == BackupStatus::BACKUP_FAILED) || (status == BackupStatus::RESTORED)
            || (status == BackupStatus::RESTORE_FAILED);
    }
    bool isErrorStatus(BackupStatus status)
    {
        return (status == BackupStatus::BACKUP_FAILED) || (status == BackupStatus::RESTORE_FAILED);
    }
    
    size_t getNumActiveBackupsChange(BackupStatus status)
    {
        return status == BackupStatus::CREATING_BACKUP;
    }
    
    size_t getNumActiveRestoresChange(BackupStatus status)
    {
        return status == BackupStatus::RESTORING;
    }
}
BackupsWorker::BackupsWorker(size_t num_backup_threads, size_t num_restore_threads)
    : backups_thread_pool(num_backup_threads,  0, num_backup_threads)
    , restores_thread_pool(num_restore_threads,  0, num_restore_threads)
    , log(&Poco::Logger::get("BackupsWorker"))
{
    
}
OperationID BackupsWorker::start(const ASTPtr & backup_or_restore_query, ContextMutablePtr context)