and uncompressed buffer pool pages.

buf_block_t: Replace page_zip, space, and offset with buf_page_t page.
Replace some integers with bit-fields.

enum buf_block_state: Rename to buf_page_state.  Add BUF_BLOCK_ZIP_PAGE.

page_zip_des_t: Add the field "state".  Make the integer fields bit-fields.

page_zip_copy(): Document which fields are copied.