Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

{
                String desired_checksums_str = zookeeper->get(fs::path(part_path) / "checksums");
                desired_checksums = MinimalisticDataPartChecksums::deserializeFrom(desired_checksums_str);
            }
            if (source_part_checksums == desired_checksums)
            {
                LOG_TRACE(log, "Found local part {} with the same checksums as {}", source_part->name, part_name);
                part_to_clone = source_part;
            }
        }
    }
    ReplicatedMergeTreeAddress address;
    ConnectionTimeouts timeouts;
    String interserver_scheme;
    InterserverCredentialsPtr credentials;
    std::optional<CurrentlySubmergingEmergingTagger> tagger_ptr;
    std::function<MutableDataPartPtr()> get_part;
    if (part_to_clone)
    {
        get_part = [&, part_to_clone]()
        {
            return cloneAndLoadDataPartOnSameDisk(part_to_clone, "tmp_clone_", part_info, metadata_snapshot);
        };
    }
    else
    {
        address.fromString(zookeeper->get(fs::path(source_replica_path) / "host"));
        timeouts = getFetchPartHTTPTimeouts(getContext());
        credentials = getContext()->getInterserverCredentials();
        interserver_scheme = getContext()->getInterserverScheme();