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. - 

Coordination::Requests ops;
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path, "", zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/metadata", metadata_str,
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/columns", metadata_snapshot->getColumns().toString(),
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/log", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/blocks", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/block_numbers", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/nonincrement_block_numbers", "",
            zkutil::CreateMode::Persistent)); 
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/leader_election", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/temp", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/replicas", "last added replica: " + replica_name,
            zkutil::CreateMode::Persistent));
        
        ops.emplace_back(zkutil::makeCreateRequest(replica_path, "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/host", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/log_pointer", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/queue", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/parts", "",