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, e.displayText());
            }
            else if (e.code() == ErrorCodes::ABORTED)
            {
                
                LOG_INFO(log, e.message());
            }
            else if (e.code() == ErrorCodes::PART_IS_TEMPORARILY_LOCKED)
            {
                
                LOG_INFO(log, e.displayText());
                cleanup_thread.wakeup();
            }
            else
                tryLogCurrentException(log, __PRETTY_FUNCTION__);
            /** This exception will be written to the queue element, and it can be looked up using `system.replication_queue` table.
              * The thread that performs this action will sleep a few seconds after the exception.
              * See `queue.processEntry` function.
              */
            throw;
        }
        catch (...)
        {
            tryLogCurrentException(log, __PRETTY_FUNCTION__);
            throw;
        }
    });
}
std::optional<JobAndPool> StorageReplicatedMergeTree::getDataProcessingJob()