page_zip_compress(), page_zip_decompress(), page_zip_write_rec():
Note that the end marker of the modification log is 1 byte.

page_zip_find_heap_no(): Remove.

page_zip_apply_log(): Remove parameter "page".  Instead of a 2-byte offset
to the record, decode the heap number using a variable-length code.

page_zip_write_rec(): Check the range of heap_no with debug assertions.
Encode the heap number instead of an offset to the start of the record.