> (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.

explicit SettingFieldOptionalUUID(const Field & field)
        {
            if (field.getType() == Field::Types::Null)
            {
                value = std::nullopt;
                return;
            }
            if (field.getType() == Field::Types::String)
            {
                const String & str = field.get<const String &>();
                if (str.empty())
                {
                    value = std::nullopt;
                    return;
                }
                UUID id;
                if (tryParse(id, str))
                {
                    value = id;
                    return;
                }
            }
            throw Exception(ErrorCodes::CANNOT_PARSE_BACKUP_SETTINGS, "Cannot parse uuid from {}", field);
        }
        explicit operator Field() const { return Field(value ? toString(*value) : ""); }