> (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 SettingFieldRestoreAccessCreationMode(const Field & field)
        {
            if (field.getType() == Field::Types::String)
            {
                const String & str = field.get<const String &>();
                if (str == "1" || boost::iequals(str, "true") || boost::iequals(str, "create"))
                {
                    value = RestoreAccessCreationMode::kCreate;
                    return;
                }
                if (boost::iequals(str, "if not exists") || boost::iequals(str, "if-not-exists")
                    || boost::iequals(str, "create if not exists") || boost::iequals(str, "create-if-not-exists"))
                {
                    value = RestoreAccessCreationMode::kCreateIfNotExists;
                    return;
                }
                if (boost::iequals(str, "replace") || boost::iequals(str, "create or replace") || boost::iequals(str, "create-or-replace"))
                {
                    value = RestoreAccessCreationMode::kReplace;
                    return;
                }
            }
            if (field.getType() == Field::Types::UInt64)
            {
                UInt64 number = field.get<UInt64>();
                if (number == 1)
                {
                    value = RestoreAccessCreationMode::kCreate;
                    return;