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. - 

zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/metadata", ReplicatedMergeTreeTableMetadata(*this, metadata_snapshot).toString(),
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/columns", metadata_snapshot->getColumns().toString(),
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/metadata_version", std::to_string(metadata_version),
            zkutil::CreateMode::Persistent));
        
        ops.emplace_back(zkutil::makeSetRequest(zookeeper_path + "/replicas", "last added replica: " + replica_name, replicas_stat.version));
        Coordination::Responses responses;
        code = zookeeper->tryMulti(ops, responses);
        if (code == Coordination::Error::ZNODEEXISTS)
        {
            throw Exception("Replica " + replica_path + " already exists.", ErrorCodes::REPLICA_IS_ALREADY_EXIST);
        }
        else if (code == Coordination::Error::ZBADVERSION)
        {
            LOG_ERROR(log, "Retrying createReplica(), because some other replicas were created at the same time");
        }
        else if (code == Coordination::Error::ZNONODE)
        {
            throw Exception("Table " + zookeeper_path + " was suddenly removed.", ErrorCodes::ALL_REPLICAS_LOST);
        }
        else
        {
            zkutil::KeeperMultiException::check(code, ops, responses);
        }
    } while (code == Coordination::Error::ZBADVERSION);
}
void StorageReplicatedMergeTree::drop()