**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 :)

&& MergeTreeSettings::isReadonlySetting(setting_name))
            {
                throw Exception{"Setting '" + setting_name + "' is readonly for storage '" + getName() + "'",
                                 ErrorCodes::READONLY_SETTING};
            }
            if (!current_value && MergeTreeSettings::isPartFormatSetting(setting_name))
            {
                MergeTreeSettings copy = *getSettings();
                copy.applyChange(changed_setting);
                String reason;
                if (!canUsePolymorphicParts(copy, &reason) && !reason.empty())
                    throw Exception("Can't change settings. Reason: " + reason, ErrorCodes::NOT_IMPLEMENTED);
            }
            if (setting_name == "storage_policy")
                checkStoragePolicy(getContext()->getStoragePolicy(new_value.safeGet<String>()));
        }
        
        for (const auto & current_setting : current_changes)
        {
            const auto & setting_name = current_setting.name;
            const Field * new_value = new_changes.tryGet(setting_name);
            
            if (MergeTreeSettings::isReadonlySetting(setting_name) && !new_value)
            {
                throw Exception{"Setting '" + setting_name + "' is readonly for storage '" + getName() + "'",
                                ErrorCodes::READONLY_SETTING};
            }
            if (MergeTreeSettings::isPartFormatSetting(setting_name) && !new_value)
            {