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. - 

{
    auto callback = [this]()
    {
        LOG_INFO(log, "Became leader");
        is_leader = true;
        merge_selecting_task->activateAndSchedule();
    };
    try
    {
        leader_election = std::make_shared<zkutil::LeaderElection>(
            getContext()->getSchedulePool(),
            fs::path(zookeeper_path) / "leader_election",
            *current_zookeeper,    
                                   
            callback,
            replica_name);
    }
    catch (...)
    {
        leader_election = nullptr;
        throw;
    }
}
void StorageReplicatedMergeTree::exitLeaderElection()