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. - 

MutableDataPartPtr & part, SimpleIncrement * increment, Transaction * out_transaction,
    std::unique_lock<std::mutex> & lock, DataPartsVector * out_covered_parts, MergeTreeDeduplicationLog * deduplication_log)
{
    if (out_transaction && &out_transaction->data != this)
        throw Exception("MergeTreeData::Transaction for one table cannot be used with another. It is a bug.",
            ErrorCodes::LOGICAL_ERROR);
    part->assertState({DataPartState::Temporary});
    MergeTreePartInfo part_info = part->info;
    String part_name;
    if (DataPartPtr existing_part_in_partition = getAnyPartInPartition(part->info.partition_id, lock))
    {
        if (part->partition.value != existing_part_in_partition->partition.value)
            throw Exception(
                "Partition value mismatch between two parts with the same partition ID. Existing part: "
                + existing_part_in_partition->name + ", newly added part: " + part->name,
                ErrorCodes::CORRUPTED_DATA);
    }
    /** It is important that obtaining new block number and adding that block to parts set is done atomically.