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 StorageInMemoryMetadata & new_metadata, const StorageInMemoryMetadata & old_metadata, bool attach) const
{
    if (!new_metadata.sorting_key.definition_ast)
        throw Exception("ORDER BY cannot be empty", ErrorCodes::BAD_ARGUMENTS);
    KeyDescription new_sorting_key = new_metadata.sorting_key;
    KeyDescription new_primary_key = new_metadata.primary_key;
    size_t sorting_key_size = new_sorting_key.column_names.size();
    size_t primary_key_size = new_primary_key.column_names.size();
    if (primary_key_size > sorting_key_size)
        throw Exception("Primary key must be a prefix of the sorting key, but its length: "
            + toString(primary_key_size) + " is greater than the sorting key length: " + toString(sorting_key_size),
            ErrorCodes::BAD_ARGUMENTS);
    NameSet primary_key_columns_set;
    for (size_t i = 0; i < sorting_key_size; ++i)
    {
        const String & sorting_key_column = new_sorting_key.column_names[i];
        if (i < primary_key_size)
        {
            const String & pk_column = new_primary_key.column_names[i];
            if (pk_column != sorting_key_column)
                throw Exception("Primary key must be a prefix of the sorting key, but the column in the position "
                    + toString(i) + " is " + sorting_key_column +", not " + pk_column,
                    ErrorCodes::BAD_ARGUMENTS);
            if (!primary_key_columns_set.emplace(pk_column).second)
                throw Exception("Primary key contains duplicate columns", ErrorCodes::BAD_ARGUMENTS);
        }
    }
    auto all_columns = new_metadata.columns.getAllPhysical();