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. - 

{
    if (!canUseAdaptiveGranularity())
    {
        if (out_reason && (settings.min_rows_for_wide_part != 0 || settings.min_bytes_for_wide_part != 0
            || settings.min_rows_for_compact_part != 0 || settings.min_bytes_for_compact_part != 0))
        {
            *out_reason = fmt::format(
                    "Table can't create parts with adaptive granularity, but settings"
                    " min_rows_for_wide_part = {}"
                    ", min_bytes_for_wide_part = {}"
                    ", min_rows_for_compact_part = {}"
                    ", min_bytes_for_compact_part = {}"
                    ". Parts with non-adaptive granularity can be stored only in Wide (default) format.",
                    settings.min_rows_for_wide_part,    settings.min_bytes_for_wide_part,
                    settings.min_rows_for_compact_part, settings.min_bytes_for_compact_part);
        }
        return false;
    }
    return true;
}
MergeTreeData::AlterConversions MergeTreeData::getAlterConversionsForPart(const MergeTreeDataPartPtr part) const