operation succeeds.

page_zip_dir_set(): Remove, as the slots will be written to a temporary
buffer.

page_zip_dir_encode(): Replace page_zip with buf.  Make some assertions
more accurate.  Write the dense page directory to buf.

page_zip_compress(): Allocate a temporary buffer for the compressed page
except the page header.  Make some assertions more accurate.

page_zip_compress(), page_zip_decompress(): Remove 2 bytes of empty space
adjacent to the area reserved for storing uncompressed columns.