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. - 

{
    std::lock_guard lock(query_id_set_mutex);
    if (query_id_set.find(query_id) == query_id_set.end())
        LOG_WARNING(log, "We have query_id removed but it's not recorded. This is a bug");
    else
        query_id_set.erase(query_id);
}
ReservationPtr MergeTreeData::balancedReservation(
    const StorageMetadataPtr & metadata_snapshot,
    size_t part_size,
    size_t max_volume_index,
    const String & part_name,
    const MergeTreePartInfo & part_info,
    MergeTreeData::DataPartsVector covered_parts,
    std::optional<CurrentlySubmergingEmergingTagger> * tagger_ptr,
    const IMergeTreeDataPart::TTLInfos * ttl_infos,
    bool is_insert)
{
    ReservationPtr reserved_space;
    auto min_bytes_to_rebalance_partition_over_jbod = getSettings()->min_bytes_to_rebalance_partition_over_jbod;
    if (tagger_ptr && min_bytes_to_rebalance_partition_over_jbod > 0 && part_size >= min_bytes_to_rebalance_partition_over_jbod)
    try
    {
        const auto & disks = getStoragePolicy()->getVolume(max_volume_index)->getDisks();
        std::map<String, size_t> disk_occupation;