Add function parameters and TODO comments regarding that.

page_dir_slot_set_n_owned(): Check page_zip_available().
page_dir_add_slots(): Add parameter page_zip.
btr_attach_half_pages(): Add parameter page_zip.
btr_node_ptr_set_child_page_no(): Add parameter page_zip.
Make comments about page_zip modification log usage more accurate.

page_dir_set_n_slots(): Add debug assertion that there will be enough space
available in the modification log of the compressed page.
page_trailer_get_len(): Simplify the formula.  Make more use of the function.

page_set_max_trx_id(), page_update_max_trx_id(): Add parameter page_zip.

Check that page_zip_decompress() is never called after modifying the
header or trailer of the compressed page, i.e., that page_zip_decompress()
will restore the uncompressed page as it was before the failed operation.

page_move_rec_list_start(): Restored.  Reorganize old page if compressed.
btr_page_split_and_insert(): Do left page splits.

buf_flush_buffered_writes(): Remove bogus assertion failure.

page_header_reset_last_insert(): Add page_zip parameter.
rec_get_next_ptr(): Return a pointer to the same page, or NULL.
page_dir_delete_slot(): Fix off-by-one errors.

Fix some (not all) bugs found by testing.

Fix bugs found during testing.

Add page_zip parameter to page_dir_slot_set_rec().
Improve the documentation of the page_zip parameters in page0page.
Add function page_trailer_get_len().

Slightly improve error handling.

Implement page_zip_write_header().
Add page_zip parameter to btr_page_set_ functions.
Allow page_copy_rec_list_end() to be called with invalid new_page_zip
if the page has been newly created.

Initial import of the zip-like compression of B-tree pages.