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. - 

{
            if (query_context->getSettingsRef().replication_alter_partitions_sync == 1)
                waitForReplicaToProcessLogEntry(replica_name, entry);
            else
                waitForAllReplicasToProcessLogEntry(entry);
        }
    }
    if (!drop_part)
    {
        String partition_id = getPartitionIDFromQuery(partition, query_context);
        cleanLastPartNode(partition_id);
    }
}
void StorageReplicatedMergeTree::truncate(
    const ASTPtr &, const StorageMetadataPtr &, ContextPtr query_context, TableExclusiveLockHolder & table_lock)
{
    table_lock.release();   
    assertNotReadonly();
    if (!is_leader)
        throw Exception("TRUNCATE cannot be done on this replica because it is not a leader", ErrorCodes::NOT_A_LEADER);
    zkutil::ZooKeeperPtr zookeeper = getZooKeeper();
    Strings partitions = zookeeper->getChildren(fs::path(zookeeper_path) / "block_numbers");
    for (String & partition_id : partitions)
    {
        LogEntry entry;
        if (dropAllPartsInPartition(*zookeeper, partition_id, entry, query_context, false))
            waitForAllReplicasToProcessLogEntry(entry);
    }
}
PartitionCommandsResultInfo StorageReplicatedMergeTree::attachPartition(