page_zip_assertion() failure in page_copy_rec_list_end().

btr_root_raise_and_insert(), btr_lift_page_up(), btr_compress():
Add page_zip_validate() assertions.

btr_compress(): Only copy FIL_PAGE_PREV when UNIV_BTR_DEBUG is defined.

page_cur_delete_rec(): Document why the page_zip_validate() assertion
was removed.

page_copy_rec_list_end(): Remove the page_zip_validate() assertion and
document why.

page_move_rec_list_end(): Add page_zip_validate() assertion.