ibuf_read_merge_pages(): Request a possibly freed page.
The change buffer is discarded lazily for freed pages either
by this function or when buf_page_create() reuses a page.

buf_page_get_low(): Relax a debug assertion.
Do not attempt change buffer merge on freed pages.

ibuf_merge_or_delete_for_page(): Assert that the page state is NORMAL.
INIT_ON_FLUSH is not possible, because in that case buf_page_create()
should have removed any buffered changes for the page.

buf_page_get_gen(): Apply buffered changes also in the case when
we can avoid reading the page based on buffered redo log records.
This addresses a hard-to-reproduce scenario that was broken in
commit 6697135c.