buf_page_print(): Print also compressed pages.  Add parameter zip_size.

buf_flush_init_for_writing(): Stamp the fields on a compressed B-tree index
page.

Add the header field FIL_PAGE_ZBLOB_SPACE_ID as an alias of FIL_PAGE_PREV.

page_zip_calc_checksum(): New function.

page_zip_compress(): Avoid copying the fields that are written in
buf_flush_init_for_writing().

page_zip_header_cmp(): New function for comparing those fields of the
page header that will not be written in buf_flush_init_for_writing().