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 getMaxPartsCountForPartitionWithState(DataPartState::Committed);
}
size_t MergeTreeData::getMaxInactivePartsCountForPartition() const
{
    return getMaxPartsCountForPartitionWithState(DataPartState::Outdated);
}
std::optional<Int64> MergeTreeData::getMinPartDataVersion() const
{
    auto lock = lockParts();
    std::optional<Int64> result;
    for (const auto & part : getDataPartsStateRange(DataPartState::Committed))
    {
        if (!result || *result > part->info.getDataVersion())
            result = part->info.getDataVersion();
    }
    return result;
}
void MergeTreeData::delayInsertOrThrowIfNeeded(Poco::Event * until) const