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. - 

MutableDataPartPtr & part, SimpleIncrement * increment, Transaction * out_transaction, MergeTreeDeduplicationLog * deduplication_log)
{
    if (out_transaction && &out_transaction->data != this)
        throw Exception("MergeTreeData::Transaction for one table cannot be used with another. It is a bug.",
            ErrorCodes::LOGICAL_ERROR);
    DataPartsVector covered_parts;
    {
        auto lock = lockParts();
        renameTempPartAndReplace(part, increment, out_transaction, lock, &covered_parts, deduplication_log);
    }
    return covered_parts;
}
void MergeTreeData::removePartsFromWorkingSet(const MergeTreeData::DataPartsVector & remove, bool clear_without_timeout, DataPartsLock & )