> (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.

{
    std::lock_guard lock{infos_mutex};
    auto it = infos.find(id);
    if (it == infos.end())
    {
        if (throw_if_error)
           throw Exception(ErrorCodes::LOGICAL_ERROR, "Unknown backup ID {}", id);
        else
            return;
    }
    auto & info = it->second;
    auto old_status = info.status;
    info.status = status;
    if (isFinalStatus(status))
        info.end_time = std::chrono::system_clock::now();
    if (isErrorStatus(status))
    {
        info.error_message = getCurrentExceptionMessage(false);
        info.exception = std::current_exception();
    }
    num_active_backups += getNumActiveBackupsChange(status) - getNumActiveBackupsChange(old_status);
    num_active_restores += getNumActiveRestoresChange(status) - getNumActiveRestoresChange(old_status);
}
void BackupsWorker::setNumFilesAndSize(const String & id, size_t num_files, UInt64 uncompressed_size, UInt64 compressed_size)