Protect Against Low Background Pool Size in ClickHouse Configuration Changes

Protect from the cases when user may set background_pool_size to value lower than number_of_free_entries_in_pool_to_execute_mutation or number_of_free_entries_in_pool_to_lower_max_size_of_merge. In these cases ALTERs won't work or the maximum size of merge will be too limited. It will throw exception explaining what to do. This closes #10897. #12728 (alexey-milovidov).