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. - 

storage_settings_ptr->max_replicated_merges_in_queue, merges_and_mutations_sum);
            UInt64 max_source_part_size_for_mutation = merger_mutator.getMaxSourcePartSizeForMutation();
            bool merge_with_ttl_allowed = merges_and_mutations_queued.merges_with_ttl < storage_settings_ptr->max_replicated_merges_with_ttl_in_queue &&
                getTotalMergesWithTTLInMergeList() < storage_settings_ptr->max_number_of_merges_with_ttl_in_pool;
            FutureMergedMutatedPart future_merged_part;
            if (storage_settings.get()->assign_part_uuids)
                future_merged_part.uuid = UUIDHelpers::generateV4();
            if (max_source_parts_size_for_merge > 0 &&
                merger_mutator.selectPartsToMerge(future_merged_part, false, max_source_parts_size_for_merge, merge_pred, merge_with_ttl_allowed, nullptr) == SelectPartsDecision::SELECTED)
            {
                create_result = createLogEntryToMergeParts(
                    zookeeper,
                    future_merged_part.parts,
                    future_merged_part.name,
                    future_merged_part.uuid,
                    future_merged_part.type,
                    deduplicate,
                    deduplicate_by_columns,
                    nullptr,
                    merge_pred.getVersion(),
                    future_merged_part.merge_type);
            }
            
            else if (max_source_part_size_for_mutation > 0 && queue.countMutations() > 0
                     && merges_and_mutations_queued.mutations < storage_settings_ptr->max_replicated_mutations_in_queue)
            {
                
                DataPartsVector data_parts = getDataPartsVector();
                for (const auto & part : data_parts)
                {