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. - 

[[fallthrough]];
        case LogEntry::GET_PART:
            do_fetch = true;
            break;
        case LogEntry::MERGE_PARTS:
            
            
            do_fetch = !tryExecuteMerge(entry);
            break;
        case LogEntry::MUTATE_PART:
            
            do_fetch = !tryExecutePartMutation(entry);
            break;
        case LogEntry::ALTER_METADATA:
            return executeMetadataAlter(entry);
        case LogEntry::SYNC_PINNED_PART_UUIDS:
            syncPinnedPartUUIDs();
            return true;
        case LogEntry::CLONE_PART_FROM_SHARD:
            executeClonePartFromShard(entry);
            return true;
        default:
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected log entry type: {}", static_cast<int>(entry.type));
    }
    if (do_fetch)
        return executeFetch(entry);
    return true;
}
bool StorageReplicatedMergeTree::tryExecuteMerge(const LogEntry & entry)