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. - 

LOG_INFO(log, "No active replica has part {}. Will fetch merged part instead.", entry.new_part_name);
                return false;
            }
            /** If no active replica has a part, and there is no merge in the queue with its participation,
              * check to see if any (active or inactive) replica has such a part or covering it.
              */
            if (replica.empty())
                enqueuePartForCheck(entry.new_part_name);
        }
        catch (...)
        {
            tryLogCurrentException(log, __PRETTY_FUNCTION__);
        }
        throw;
    }
    return true;
}
bool StorageReplicatedMergeTree::executeFetchShared(
    const String & source_replica,
    const String & new_part_name,
    const DiskPtr & disk,
    const String & path)
{
    if (source_replica.empty())
    {
        LOG_INFO(log, "No active replica has part {} on S3.", new_part_name);
        return false;
    }
    const auto storage_settings_ptr = getSettings();
    auto metadata_snapshot = getInMemoryMetadataPtr();