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. - 

{
    assertNotReadonly();
    if (!is_leader)
        throw Exception("DROP PART|PARTITION cannot be done on this replica because it is not a leader", ErrorCodes::NOT_A_LEADER);
    zkutil::ZooKeeperPtr zookeeper = getZooKeeper();
    LogEntry entry;
    bool did_drop;
    if (drop_part)
    {
        String part_name = partition->as<ASTLiteral &>().value.safeGet<String>();
        did_drop = dropPart(zookeeper, part_name, entry, detach, throw_if_noop);
    }
    else
    {
        String partition_id = getPartitionIDFromQuery(partition, query_context);
        did_drop = dropAllPartsInPartition(*zookeeper, partition_id, entry, query_context, detach);
    }
    if (did_drop)
    {
        
        if (query_context->getSettingsRef().replication_alter_partitions_sync != 0)