**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 :)

bool is_covered_by_min_max_block = part->info.min_block <= drop_range.min_block && part->info.max_block >= drop_range.max_block && part->info.getMutationVersion() >= drop_range.getMutationVersion();
            if (is_covered_by_min_max_block)
            {
                LOG_INFO(log, "Skipping drop range for part {} because covering part {} already exists", drop_range.getPartName(), part->name);
                return {};
            }
        }
        if (part->info.min_block < drop_range.min_block)
        {
            if (drop_range.min_block <= part->info.max_block)
            {
                
                throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected merged part {} intersecting drop range {}",
                                part->name, drop_range.getPartName());
            }
            continue;
        }
        
        if (part->info.min_block > drop_range.max_block)
            break;
        if (part->info.min_block <= drop_range.max_block && drop_range.max_block < part->info.max_block)
        {
            
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected merged part {} intersecting drop range {}",