> (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.

{
    if (storage_id.uuid != UUIDHelpers::Nil)
        return storage_id;
    if (!storage_id)
    {
        if (exception)
            exception->emplace("Both table name and UUID are empty", ErrorCodes::UNKNOWN_TABLE);
        return storage_id;
    }
    bool look_for_external_table = where & StorageNamespace::ResolveExternal;
    
    if (isGlobalContext())
        look_for_external_table = false;
    bool in_current_database = where & StorageNamespace::ResolveCurrentDatabase;
    bool in_specified_database = where & StorageNamespace::ResolveGlobal;
    if (!storage_id.database_name.empty())
    {
        if (in_specified_database)
            return storage_id;     
        if (exception)