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. - 

{
    auto lock = lockParts();
    return getActiveContainingPart(part_info, DataPartState::Committed, lock);
}
MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const String & part_name) const
{
    auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    return getActiveContainingPart(part_info);
}
MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorInPartition(MergeTreeData::DataPartState state, const String & partition_id)
{
    DataPartStateAndPartitionID state_with_partition{state, partition_id};
    auto lock = lockParts();
    return DataPartsVector(
        data_parts_by_state_and_info.lower_bound(state_with_partition),
        data_parts_by_state_and_info.upper_bound(state_with_partition));
}
MergeTreeData::DataPartPtr MergeTreeData::getPartIfExists(const MergeTreePartInfo & part_info, const MergeTreeData::DataPartStates & valid_states)