**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 ASTPtr & new_settings,
        AlterLockHolder & )
{
    if (new_settings)
    {
        bool has_storage_policy_changed = false;
        const auto & new_changes = new_settings->as<const ASTSetQuery &>().changes;
        for (const auto & change : new_changes)
        {
            if (change.name == "storage_policy")
            {
                StoragePolicyPtr new_storage_policy = getContext()->getStoragePolicy(change.value.safeGet<String>());
                StoragePolicyPtr old_storage_policy = getStoragePolicy();
                
                if (new_storage_policy != old_storage_policy)
                {
                    checkStoragePolicy(new_storage_policy);
                    std::unordered_set<String> all_diff_disk_names;
                    for (const auto & disk : new_storage_policy->getDisks())
                        all_diff_disk_names.insert(disk->getName());
                    for (const auto & disk : old_storage_policy->getDisks())
                        all_diff_disk_names.erase(disk->getName());