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. - 

zkutil::ZooKeeperPtr & zookeeper, String part_name, LogEntry & entry, bool detach, bool throw_if_noop)
{
    LOG_TRACE(log, "Will try to insert a log entry to DROP_RANGE for part: " + part_name);
    auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    while (true)
    {
        ReplicatedMergeTreeMergePredicate merge_pred = queue.getMergePredicate(zookeeper);
        auto part = getPartIfExists(part_info, {MergeTreeDataPartState::Committed});
        if (!part)
        {
            if (throw_if_noop)
                throw Exception("Part " + part_name + " not found locally, won't try to drop it.", ErrorCodes::NO_SUCH_DATA_PART);
            return false;
        }