to BUF_BLOCK_ZIP_DIRTY (discarding the uncompressed page corresponding
to a modified compressed page that has not been flushed to disk).

buf_page_can_relocate(): New function, modelled after
buf_flush_ready_for_replace().

buf_LRU_free_block(): Allow the transition to BUF_BLOCK_ZIP_DIRTY.

buf_flush_insert_into_flush_list(): Make the prototype public.

buf_buddy_relocate(): Remove an unnecessary switch statement.
Use buf_page_can_relocate() instead of buf_flush_ready_for_replace().

buf_page_peek(): Made UNIV_INLINE.

Document UNIV_ZIP_DEBUG.