> (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 restore_query = std::static_pointer_cast<ASTBackupQuery>(query->clone());
    auto restore_settings = RestoreSettings::fromRestoreQuery(*restore_query);
    
    OperationID restore_id;
    if (restore_settings.internal)
        restore_id = "internal-" + toString(UUIDHelpers::generateV4()); 
    else if (!restore_settings.id.empty())
        restore_id = restore_settings.id;
    else
        restore_id = toString(UUIDHelpers::generateV4());
    std::shared_ptr<IRestoreCoordination> restore_coordination;
    if (restore_settings.internal)