> (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.

{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have external tables");
    auto lock = getLock();
    if (external_tables_mapping.end() != external_tables_mapping.find(table_name))
        throw Exception("Temporary table " + backQuoteIfNeed(table_name) + " already exists.", ErrorCodes::TABLE_ALREADY_EXISTS);
    external_tables_mapping.emplace(table_name, std::make_shared<TemporaryTableHolder>(std::move(temporary_table)));
}
std::shared_ptr<TemporaryTableHolder> Context::removeExternalTable(const String & table_name)