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. - 

QueryPlan & query_plan,
    const Names & column_names,
    const StorageMetadataPtr & metadata_snapshot,
    SelectQueryInfo & query_info,
    ContextPtr local_context,
    QueryProcessingStage::Enum processed_stage,
    const size_t max_block_size,
    const unsigned num_streams)
{
    /** The `select_sequential_consistency` setting has two meanings:
    * 1. To throw an exception if on a replica there are not all parts which have been written down on quorum of remaining replicas.
    * 2. Do not read parts that have not yet been written to the quorum of the replicas.
    * For this you have to synchronously go to ZooKeeper.
    */
    if (local_context->getSettingsRef().select_sequential_consistency)
    {
        auto max_added_blocks = getMaxAddedBlocks();
        if (auto plan = reader.read(
                column_names, metadata_snapshot, query_info, local_context, max_block_size, num_streams, processed_stage, &max_added_blocks))
            query_plan = std::move(*plan);
        return;
    }
    if (auto plan = reader.read(column_names, metadata_snapshot, query_info, local_context, max_block_size, num_streams, processed_stage))
        query_plan = std::move(*plan);
}
Pipe StorageReplicatedMergeTree::read(