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. - 

const String & source_replica_path, DiskPtr replaced_disk, String replaced_part_path)
{
    auto zookeeper = getZooKeeper();
    const auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    if (auto part = getPartIfExists(part_info, {IMergeTreeDataPart::State::Outdated, IMergeTreeDataPart::State::Deleting}))
    {
        LOG_DEBUG(log, "Part {} should be deleted after previous attempt before fetch", part->name);
        
        cleanup_thread.wakeup();
        return false;
    }
    {
        std::lock_guard lock(currently_fetching_parts_mutex);
        if (!currently_fetching_parts.insert(part_name).second)
        {
            LOG_DEBUG(log, "Part {} is already fetching right now", part_name);
            return false;
        }
    }
    SCOPE_EXIT_MEMORY
    ({
        std::lock_guard lock(currently_fetching_parts_mutex);
        currently_fetching_parts.erase(part_name);
    });
    LOG_DEBUG(log, "Fetching part {} from {}", part_name, source_replica_path);
    TableLockHolder table_lock_holder = lockForShare(RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);