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 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)
{
    QueryPlan plan;
    read(plan, column_names, metadata_snapshot, query_info, local_context, processed_stage, max_block_size, num_streams);
    return plan.convertToPipe(
        QueryPlanOptimizationSettings::fromContext(local_context),
        BuildQueryPipelineSettings::fromContext(local_context));
}
template <class Func>
void StorageReplicatedMergeTree::foreachCommittedParts(Func && func, bool select_sequential_consistency) const