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. - 

{
    UInt64 res = 0;
    foreachCommittedParts([&res](auto & part) { res += part->rows_count; }, settings.select_sequential_consistency);
    return res;
}
std::optional<UInt64> StorageReplicatedMergeTree::totalRowsByPartitionPredicate(const SelectQueryInfo & query_info, ContextPtr local_context) const
{
    DataPartsVector parts;
    foreachCommittedParts([&](auto & part) { parts.push_back(part); }, local_context->getSettingsRef().select_sequential_consistency);
    return totalRowsByPartitionPredicateImpl(query_info, local_context, parts);
}
std::optional<UInt64> StorageReplicatedMergeTree::totalBytes(const Settings & settings) const