copy BLOB pointers.

page_zip_dir_find_free_low(): New function,
split from page_zip_dir_find_free().

Add comments about processing the records in heap_no order.

Fix some typographic errors in comments and improve formatting.

page_zip_decompress(): Initialize (clear) the BLOB pointers in deleted records.

page_zip_clear_rec(): Relocate page_zip_validate() assertions, so that they
will not fail if a record containing BLOB pointers is being deleted.

Note that page_zip_validate() will fail if page_zip_clear_rec() is unable
to clear the record.