> (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.

exception->emplace("External and temporary tables have no database, but " +
                        storage_id.database_name + " is specified", ErrorCodes::UNKNOWN_TABLE);
        return StorageID::createEmpty();
    }
    
    if (look_for_external_table)
    {
        auto resolved_id = StorageID::createEmpty();
        auto try_resolve = [&](ContextPtr context) -> bool
        {
            const auto & tables = context->external_tables_mapping;
            auto it = tables.find(storage_id.getTableName());
            if (it == tables.end())
                return false;
            resolved_id = it->second->getGlobalTableID();
            return true;
        };
        
        if (try_resolve(shared_from_this()))
            return resolved_id;
        
        auto query_context_ptr = query_context.lock();
        bool is_local_context = query_context_ptr && query_context_ptr.get() != this;
        if (is_local_context && try_resolve(query_context_ptr))
            return resolved_id;
        
        auto session_context_ptr = session_context.lock();
        bool is_local_or_query_context = session_context_ptr && session_context_ptr.get() != this;
        if (is_local_or_query_context && try_resolve(session_context_ptr))
            return resolved_id;