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. - 

entry.type = LogEntry::DROP_RANGE;
    entry.source_replica = replica_name;
    entry.new_part_name = drop_range_fake_part_name;
    entry.detach = detach;
    entry.create_time = time(nullptr);
    Coordination::Requests ops;
    ops.emplace_back(zkutil::makeCreateRequest(fs::path(zookeeper_path) / "log/log-", entry.toString(), zkutil::CreateMode::PersistentSequential));
    
    
    
    ops.emplace_back(zkutil::makeCheckRequest(alter_partition_version_path, alter_partition_version_stat.version));
    ops.emplace_back(zkutil::makeSetRequest(alter_partition_version_path, "", -1));
    
    ops.emplace_back(zkutil::makeSetRequest(fs::path(zookeeper_path) / "log", "", -1));
    delimiting_block_lock->getUnlockOps(ops);
    if (auto txn = query_context->getZooKeeperMetadataTransaction())
        txn->moveOpsTo(ops);
    Coordination::Responses responses;
    Coordination::Error code = zookeeper.tryMulti(ops, responses);
    if (code == Coordination::Error::ZOK)
        delimiting_block_lock->assumeUnlocked();
    else if (code == Coordination::Error::ZBADVERSION)
        throw Exception(ErrorCodes::CANNOT_ASSIGN_ALTER, "Cannot assign ALTER PARTITION, because another ALTER PARTITION query was concurrently executed");
    else
        zkutil::KeeperMultiException::check(code, ops, responses);
    String log_znode_path = dynamic_cast<const Coordination::CreateResponse &>(*responses.front()).path_created;
    entry.znode_name = log_znode_path.substr(log_znode_path.find_last_of('/') + 1);
    return true;
}
CheckResults StorageReplicatedMergeTree::checkData(const ASTPtr & query, ContextPtr local_context)