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 disk = getDiskForPart(part_name, additional_path);
    if (disk)
        return relative_data_path + additional_path;
    return {};
}
Strings MergeTreeData::getDataPaths() const
{
    Strings res;
    auto disks = getStoragePolicy()->getDisks();
    for (const auto & disk : disks)
        res.push_back(getFullPathOnDisk(disk));
    return res;
}
MergeTreeData::PathsWithDisks MergeTreeData::getRelativeDataPathsWithDisks() const
{
    PathsWithDisks res;
    auto disks = getStoragePolicy()->getDisks();
    for (const auto & disk : disks)
        res.emplace_back(relative_data_path, disk);
    return res;
}
MergeTreeData::MatcherFn MergeTreeData::getPartitionMatcher(const ASTPtr & partition_ast, ContextPtr local_context) const