operations on BLOB columns.  There are some bugs in the code, because
test-insert and a few other tests fail.

page_mem_free(): Add parameter index.  Decrement PAGE_N_RECS here.
Move some operations to page_zip_dir_delete().

page_zip_clear_rec(): Make this a static function.

page_zip_dir_delete(): Add parameters index and offsets.
Decrement PAGE_N_RECS and clear info_bits and n_owned.

page_zip_get_n_prev_extern(): Correct the synopsis and the algorithm.
Add parameter page_zip.  Search the records in heap_no order instead
of collation order.

page_zip_compress(), page_zip_decompress(): Only copy BLOB pointers
and increment n_blobs for records that have not been deleted.

page_zip_clear_rec(): Clear trx_id and roll_ptr on the compressed page.

page_zip_dir_delete(): Decrement PAGE_N_RECS.  Shift the array of
BLOB pointers.  Call page_zip_clear_rec().

page_zip_dir_add_slot(): Shift the array of BLOB pointers to make
space of roll_ptr and trx_id.

page_cur_delete_rec(): Do not decrement PAGE_N_RECS or call
page_zip_clear_rec(), as page_mem_free() already does it.