Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

const String & backup_name,
    ContextPtr local_context,
    TableLockHolder &)
{
    return unfreezePartitionsByMatcher([] (const String &) { return true; }, backup_name, local_context);
}
PartitionCommandsResultInfo MergeTreeData::unfreezePartitionsByMatcher(MatcherFn matcher, const String & backup_name, ContextPtr)
{
    auto backup_path = fs::path("shadow") / escapeForFileName(backup_name) / relative_data_path;
    LOG_DEBUG(log, "Unfreezing parts by path {}", backup_path.generic_string());
    PartitionCommandsResultInfo result;
    for (const auto & disk : getStoragePolicy()->getDisks())
    {
        if (!disk->exists(backup_path))
            continue;
        for (auto it = disk->iterateDirectory(backup_path); it->isValid(); it->next())
        {
            const auto & partition_directory = it->name();
            /
            auto found = partition_directory.find('_');
            if (found == std::string::npos)
                continue;
            auto partition_id = partition_directory.substr(0, found);