> (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.

{
    Info info;
    info.id = id;
    info.name = name;
    info.internal = internal;
    info.status = status;
    info.start_time = std::chrono::system_clock::now();
    if (isFinalStatus(status))
        info.end_time = info.start_time;
    std::lock_guard lock{infos_mutex};
    auto it = infos.find(id);
    if (it != infos.end())
    {
        
        auto current_status = it->second.status;
        if (!isFinalStatus(current_status))
            throw Exception(ErrorCodes::BAD_ARGUMENTS, "Cannot start a backup or restore: ID {} is already in use", id);
    }
    infos[id] = std::move(info);
    num_active_backups += getNumActiveBackupsChange(status);
    num_active_restores += getNumActiveRestoresChange(status);
}
void BackupsWorker::setStatus(const String & id, BackupStatus status, bool throw_if_error)