> (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 disk_ptr = getDisk(cache_disk_name);
    if (!disk_ptr)
        throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG, "Disk '{}' is not found", cache_disk_name);
    const auto * disk_object_storage_ptr = dynamic_cast<const DiskObjectStorage *>(disk_ptr.get());
    if (!disk_object_storage_ptr)
        throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG, "Disk '{}' does not use cache", cache_disk_name);
    auto file_cache = disk_object_storage_ptr->getCache();
    if (!file_cache)
        throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG, "Cache '{}' is not found", file_cache->getBasePath());
    LOG_DEBUG(shared->log, "Using file cache ({}) for temporary files", file_cache->getBasePath());
    shared->tmp_path = file_cache->getBasePath();
    VolumePtr volume = createLocalSingleDiskVolume(shared->tmp_path);
    shared->temp_data_on_disk = std::make_shared<TemporaryDataOnDiskScope>(volume, file_cache.get(), max_size);
}
void Context::setFlagsPath(const String & path)
{
    auto lock = getLock();
    shared->flags_path = path;
}
void Context::setUserFilesPath(const String & path)
{
    auto lock = getLock();
    shared->user_files_path = path;
}
void Context::setDictionariesLibPath(const String & path)