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. - 

{
    std::lock_guard lock(pinned_part_uuids_mutex);
    return pinned_part_uuids;
}
MergeTreeData::CurrentlyMovingPartsTagger::CurrentlyMovingPartsTagger(MergeTreeMovingParts && moving_parts_, MergeTreeData & data_)
    : parts_to_move(std::move(moving_parts_)), data(data_)
{
    for (const auto & moving_part : parts_to_move)
        if (!data.currently_moving_parts.emplace(moving_part.part).second)
            throw Exception("Cannot move part '" + moving_part.part->name + "'. It's already moving.", ErrorCodes::LOGICAL_ERROR);
}
MergeTreeData::CurrentlyMovingPartsTagger::~CurrentlyMovingPartsTagger()