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 StorageMetadataPtr & metadata_snapshot,
    bool deduplicate,
    const Names & deduplicate_by_columns,
    MergeMutateSelectedEntry & merge_mutate_entry,
    TableLockHolder & table_lock_holder)
{
    auto & future_part = merge_mutate_entry.future_part;
    Stopwatch stopwatch;
    MutableDataPartPtr new_part;
    auto table_id = getStorageID();
    auto merge_list_entry = getContext()->getMergeList().insert(table_id.database_name, table_id.table_name, future_part);
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::MERGE_PARTS,
            execution_status,