**Describe the bug**

Since we upgraded to 20.10.3.30, one of our clickhouse cluster takes much more time to start. (Up to 50 minutes, and before it took ~3 minutes)
The log file is filled with
>  <Debug> DiskLocal: Reserving 1.00 MiB on disk `default`, having unreserved 16.90 TiB

Additionally, the memory consumption increases, linearly, a lot (up to arround 230 G for us) and then drop back to normal values once clickhouse finished starting up. During that loading phase, a single CPU core is used at 100%.
If that memory is not available for the machine, clickhouse gets OOMed by the kernel.


**How to reproduce**
* Issue appeared after we upgraded to ClickHouse server version 20.10.3.30

* How could I find wich of our tables may be responsible for that ?


**Expected behavior**
A fast startup :)

{
    const auto settings = getSettings();
    if (parts_to_remove.size() > 1 && settings->max_part_removal_threads > 1 && parts_to_remove.size() > settings->concurrent_part_removal_threshold)
    {
        
        size_t num_threads = std::min<size_t>(settings->max_part_removal_threads, parts_to_remove.size());
        std::mutex part_names_mutex;
        ThreadPool pool(num_threads);
        
        for (const DataPartPtr & part : parts_to_remove)
        {
            pool.scheduleOrThrowOnError([&, thread_group = CurrentThread::getGroup()]
            {
                if (thread_group)
                    CurrentThread::attachToIfDetached(thread_group);
                LOG_DEBUG(log, "Removing part from filesystem {}", part->name);
                part->remove();
                if (part_names_successed)
                {
                    std::lock_guard lock(part_names_mutex);
                    part_names_successed->insert(part->name);