Refactor spider_db_append_key_where_internal()

Refactor the function spider_db_append_key_where_internal(). The function is quite large and difficult to maintain. It was a source of some bugs that wrongly rewrite queries (e.g., MDEV-25985, MDEV-26111).

• Split the function into multiple different functions by that sql_type is SQL or HANDLER and that two key_ranges are NULL or not.
• * (other refactoring here)

You don't need to make everything ideal at once, and it's impossible to do that with complex middleware like databases. Refactor only to the extent that you are sure it is safe to do so without additional unit tests.