> (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.

{
        case QueryLogFactories::AggregateFunction:
            query_factories_info.aggregate_functions.emplace(created_object);
            break;
        case QueryLogFactories::AggregateFunctionCombinator:
            query_factories_info.aggregate_function_combinators.emplace(created_object);
            break;
        case QueryLogFactories::Database:
            query_factories_info.database_engines.emplace(created_object);
            break;
        case QueryLogFactories::DataType:
            query_factories_info.data_type_families.emplace(created_object);
            break;
        case QueryLogFactories::Dictionary:
            query_factories_info.dictionaries.emplace(created_object);
            break;
        case QueryLogFactories::Format:
            query_factories_info.formats.emplace(created_object);
            break;
        case QueryLogFactories::Function:
            query_factories_info.functions.emplace(created_object);
            break;
        case QueryLogFactories::Storage:
            query_factories_info.storages.emplace(created_object);
            break;
        case QueryLogFactories::TableFunction:
            query_factories_info.table_functions.emplace(created_object);
    }
}
StoragePtr Context::executeTableFunction(const ASTPtr & table_expression, const ASTSelectQuery * select_query_hint)