> (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.

UInt64 number = field.get<UInt64>();
                if (number == 1)
                {
                    value = RestoreTableCreationMode::kCreate;
                    return;
                }
                if (number == 0)
                {
                    value = RestoreTableCreationMode::kMustExist;
                    return;
                }
            }
            throw Exception(ErrorCodes::CANNOT_PARSE_BACKUP_SETTINGS, "Cannot parse creation mode from {}", field);
        }
        explicit operator Field() const
        {
            switch (value)
            {
                case RestoreTableCreationMode::kCreate: return Field{true};
                case RestoreTableCreationMode::kMustExist: return Field{false};
                case RestoreTableCreationMode::kCreateIfNotExists: return Field{"if-not-exists"};
            }
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected value of enum RestoreTableCreationMode: {}", static_cast<int>(value));
        }
    };
    using SettingFieldRestoreDatabaseCreationMode = SettingFieldRestoreTableCreationMode;
    struct SettingFieldRestoreAccessCreationMode
    {
        RestoreAccessCreationMode value;
        explicit SettingFieldRestoreAccessCreationMode(RestoreAccessCreationMode value_) : value(value_) {}