btr_lift_page_up(): Invoke page_zip_validate() on every page whose level
is adjusted.

btr_compress(): Invoke page_zip_validate() on merge_page at the end.

page_zip_copy_recs(): Relax the page_zip_validate(...) to
page_zip_validate_low(..., sloppy = TRUE) to avoid bogus assertion failures.

page_copy_rec_list_end(), page_delete_rec_list_start():
Invoke page_zip_validate_low(sloppy = TRUE).