> (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.

{
    auto it = special_scalars.find(name);
    if (special_scalars.end() == it)
        return nullptr;
    return &it->second;
}
Tables Context::getExternalTables() const
{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have external tables");
    auto lock = getLock();
    Tables res;
    for (const auto & table : external_tables_mapping)
        res[table.first] = table.second->getTable();
    auto query_context_ptr = query_context.lock();
    auto session_context_ptr = session_context.lock();
    if (query_context_ptr && query_context_ptr.get() != this)
    {
        Tables buf = query_context_ptr->getExternalTables();
        res.insert(buf.begin(), buf.end());
    }
    else if (session_context_ptr && session_context_ptr.get() != this)
    {
        Tables buf = session_context_ptr->getExternalTables();
        res.insert(buf.begin(), buf.end());
    }
    return res;
}
void Context::addExternalTable(const String & table_name, TemporaryTableHolder && temporary_table)