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. - 

{
    String part_path = fs::path(replica_path) / "parts" / part_name;
    if (has_children)
    {
        ops.emplace_back(zkutil::makeRemoveRequest(fs::path(part_path) / "checksums", -1));
        ops.emplace_back(zkutil::makeRemoveRequest(fs::path(part_path) / "columns", -1));
    }
    ops.emplace_back(zkutil::makeRemoveRequest(part_path, -1));
}
void StorageReplicatedMergeTree::removePartFromZooKeeper(const String & part_name)
{
    auto zookeeper = getZooKeeper();
    String part_path = fs::path(replica_path) / "parts" / part_name;
    Coordination::Stat stat;
    
    if (!zookeeper->exists(part_path, &stat))
        return;
    Coordination::Requests ops;
    removePartFromZooKeeper(part_name, ops, stat.numChildren > 0);
    zookeeper->multi(ops);
}
void StorageReplicatedMergeTree::removePartAndEnqueueFetch(const String & part_name)