> (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.

{
        
        OperationID id;
        
        String name;
        
        bool internal = false;
        
        BackupStatus status;
        
        size_t num_files = 0;
        
        UInt64 uncompressed_size = 0;
        
        UInt64 compressed_size = 0;
        
        std::exception_ptr exception;
        String error_message;
        std::chrono::system_clock::time_point start_time;
        std::chrono::system_clock::time_point end_time;
    };
    Info getInfo(const OperationID & id) const;
    std::vector<Info> getAllInfos() const;
private:
    OperationID startMakingBackup(const ASTPtr & query, const ContextPtr & context);
    void doBackup(
        const std::shared_ptr<ASTBackupQuery> & backup_query,
        const OperationID & backup_id,
        const String & backup_name_for_logging,
        const BackupInfo & backup_info,