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. - 

{
    
    if (queue.actions_blocker.isCancelled())
        return {};
    
    ReplicatedMergeTreeQueue::SelectedEntryPtr selected_entry = selectQueueEntry();
    if (!selected_entry)
        return {};
    PoolType pool_type;
    
    if (selected_entry->log_entry->type == LogEntry::GET_PART)
        pool_type = PoolType::FETCH;
    else
        pool_type = PoolType::MERGE_MUTATE;
    return JobAndPool{[this, selected_entry] () mutable
    {
        return processQueueEntry(selected_entry);
    }, pool_type};
}
bool StorageReplicatedMergeTree::canExecuteFetch(const ReplicatedMergeTreeLogEntry & entry, String & disable_reason) const