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. - 

{
    const auto settings = getSettings();
    const size_t parts_count_in_total = getPartsCount();
    if (parts_count_in_total >= settings->max_parts_in_total)
    {
        ProfileEvents::increment(ProfileEvents::RejectedInserts);
        throw Exception("Too many parts (" + toString(parts_count_in_total) + ") in all partitions in total. This indicates wrong choice of partition key. The threshold can be modified with 'max_parts_in_total' setting in <merge_tree> element in config.xml or with per-table setting.", ErrorCodes::TOO_MANY_PARTS);
    }
    size_t parts_count_in_partition = getMaxPartsCountForPartition();
    ssize_t k_inactive = -1;
    if (settings->inactive_parts_to_throw_insert > 0 || settings->inactive_parts_to_delay_insert > 0)
    {
        size_t inactive_parts_count_in_partition = getMaxInactivePartsCountForPartition();
        if (settings->inactive_parts_to_throw_insert > 0 && inactive_parts_count_in_partition >= settings->inactive_parts_to_throw_insert)
        {
            ProfileEvents::increment(ProfileEvents::RejectedInserts);
            throw Exception(
                ErrorCodes::TOO_MANY_PARTS,
                "Too many inactive parts ({}). Parts cleaning are processing significantly slower than inserts",
                inactive_parts_count_in_partition);
        }
        k_inactive = ssize_t(inactive_parts_count_in_partition) - ssize_t(settings->inactive_parts_to_delay_insert);
    }
    if (parts_count_in_partition >= settings->parts_to_throw_insert)
    {
        ProfileEvents::increment(ProfileEvents::RejectedInserts);
        throw Exception(