> (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())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Unknown backup ID {}", id);
    auto & info = it->second;
    info.num_files = num_files;
    info.uncompressed_size = uncompressed_size;
    info.compressed_size = compressed_size;
}
void BackupsWorker::wait(const OperationID & id, bool rethrow_exception)
{
    std::unique_lock lock{infos_mutex};
    status_changed.wait(lock, [&]
    {
        auto it = infos.find(id);
        if (it == infos.end())
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Unknown backup ID {}", id);
        const auto & info = it->second;
        auto current_status = info.status;
        if (rethrow_exception && isErrorStatus(current_status))
            std::rethrow_exception(info.exception);
        return isFinalStatus(current_status);
    });
}
BackupsWorker::Info BackupsWorker::getInfo(const OperationID & id) const