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. - 

{
    return getDataPartsVector({DataPartState::Committed});
}
MergeTreeData::DataPartPtr MergeTreeData::getAnyPartInPartition(
    const String & partition_id, DataPartsLock & / const
{
    auto it = data_parts_by_state_and_info.lower_bound(DataPartStateAndPartitionID{DataPartState::Committed, partition_id});
    if (it != data_parts_by_state_and_info.end() && (*it)->getState() == DataPartState::Committed && (*it)->info.partition_id == partition_id)
        return *it;
    return nullptr;
}
void MergeTreeData::Transaction::rollbackPartsToTemporaryState()
{
    if (!isEmpty())
    {
        WriteBufferFromOwnString buf;
        buf << " Rollbacking parts state to temporary and removing from working set:";
        for (const auto & part : precommitted_parts)
            buf << " " << part->relative_path;
        buf << ".";
        LOG_DEBUG(data.log, "Undoing transaction.{}", buf.str());
        data.removePartsFromWorkingSetImmediatelyAndSetTemporaryState(
            DataPartsVector(precommitted_parts.begin(), precommitted_parts.end()));
    }
    clear();
}
void MergeTreeData::Transaction::rollback()