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. - 

const String & zookeeper_path, const zkutil::EphemeralNodeHolder::Ptr & metadata_drop_lock, Poco::Logger * logger)
{
    bool completely_removed = false;
    Strings children;
    Coordination::Error code = zookeeper->tryGetChildren(zookeeper_path, children);
    if (code == Coordination::Error::ZNONODE)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "There is a race condition between creation and removal of replicated table. It's a bug");
    for (const auto & child : children)
        if (child != "dropped")
            zookeeper->tryRemoveRecursive(fs::path(zookeeper_path) / child);
    Coordination::Requests ops;
    Coordination::Responses responses;
    ops.emplace_back(zkutil::makeRemoveRequest(metadata_drop_lock->getPath(), -1));
    ops.emplace_back(zkutil::makeRemoveRequest(fs::path(zookeeper_path) / "dropped", -1));
    ops.emplace_back(zkutil::makeRemoveRequest(zookeeper_path, -1));
    code = zookeeper->tryMulti(ops, responses);
    if (code == Coordination::Error::ZNONODE)
    {
        throw Exception(ErrorCodes::LOGICAL_ERROR, "There is a race condition between creation and removal of replicated table. It's a bug");
    }
    else if (code == Coordination::Error::ZNOTEMPTY)
    {
        LOG_ERROR(logger, "Table was not completely removed from ZooKeeper, {} still exists and may contain some garbage,"
                          "but someone is removing it right now.", zookeeper_path);
    }
    else if (code != Coordination::Error::ZOK)
    {