**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 :)

{
    LOG_DEBUG(log, "Loading data parts");
    auto metadata_snapshot = getInMemoryMetadataPtr();
    const auto settings = getSettings();
    MutableDataPartsVector parts_from_wal;
    Strings part_file_names;
    auto disks = getStoragePolicy()->getDisks();
    
    if (!getStoragePolicy()->isDefaultPolicy() && !skip_sanity_checks)
    {
        
        std::unordered_set<String> defined_disk_names;
        
        std::unordered_set<String> disk_names_wrapped_in_cache;
        for (const auto & disk_ptr : disks)
            defined_disk_names.insert(disk_ptr->getName());
        for (const auto & [_, disk_ptr] : getContext()->getDisksMap())
        {