> (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 ASTBackupQuery & backup_query = typeid_cast<const ASTBackupQuery &>(*backup_or_restore_query);
    if (backup_query.kind == ASTBackupQuery::Kind::BACKUP)
        return startMakingBackup(backup_or_restore_query, context);
    else
        return startRestoring(backup_or_restore_query, context);
}
OperationID BackupsWorker::startMakingBackup(const ASTPtr & query, const ContextPtr & context)
{
    auto backup_query = std::static_pointer_cast<ASTBackupQuery>(query->clone());
    auto backup_settings = BackupSettings::fromBackupQuery(*backup_query);
    if (!backup_settings.backup_uuid)
        backup_settings.backup_uuid = UUIDHelpers::generateV4();
    
    OperationID backup_id;
    if (backup_settings.internal)
        backup_id = "internal-" + toString(UUIDHelpers::generateV4()); 
    else if (!backup_settings.id.empty())
        backup_id = backup_settings.id;
    else
        backup_id = toString(*backup_settings.backup_uuid);
    std::shared_ptr<IBackupCoordination> backup_coordination;
    if (backup_settings.internal)
    {