> (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 lock = getLock();
    if (!shared->system_logs)
        return {};
    return shared->system_logs->query_views_log;
}
std::shared_ptr<PartLog> Context::getPartLog(const String & part_database) const
{
    auto lock = getLock();
    
    if (!shared->system_logs)
        return {};
    
    
    
    if (part_database == DatabaseCatalog::SYSTEM_DATABASE)
        return {};
    return shared->system_logs->part_log;
}
std::shared_ptr<TraceLog> Context::getTraceLog() const
{
    auto lock = getLock();
    if (!shared->system_logs)
        return {};
    return shared->system_logs->trace_log;
}
std::shared_ptr<TextLog> Context::getTextLog() const