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 auto storage_settings_ptr = getSettings();
    return storage_settings_ptr->index_granularity_bytes != 0 &&
        (storage_settings_ptr->enable_mixed_granularity_parts ||
            (!has_non_adaptive_index_granularity_parts && !other_replicas_fixed_granularity));
}
MutationCommands StorageReplicatedMergeTree::getFirstAlterMutationCommandsForPart(const DataPartPtr & part) const
{
    return queue.getFirstAlterMutationCommandsForPart(part);
}
void StorageReplicatedMergeTree::startBackgroundMovesIfNeeded()