A page with multiple records or deleted records still does not compress
or decompress properly.
buf_flush_init_for_writing(): Initialize block->page_zip properly so that all
assertions in page0zip can be enabled.
page_zip_decompress(): Note that corrupt data should not lead to assertions.
page_zip_dir_set(): Correct the interface. Fix off-by-one error.
page_zip_dir_get(): Fix off-by-one error.
page0zip.c: Replace n_heap with n_dense and add comments about
the infimum and supremum records whenever we subtract 2 from heap_no.
Fix some programming errors.
buf0flu.c: Allocate the temporary buffer from buf_frame_alloc().
page_zip_simple_validate(): Do not assert page_zip->m_start >= PAGE_DATA.
page_zip_compress(): Replace some assertions with page_zip_simple_validate(),
and do not assert anything about page_zip->data contents.
page_zip_validate(): Do not compare the page trailer bytes.
page_zip_write(): Assert that the entire page headers match and
that page_zip->m_start >= PAGE_DATA.
