page_zip_alloc(): Add parameter "mtr" and log successful calls
to page_zip_compress().

page_zip_write_blob_ptr(), page_zip_write_node_ptr(): Write the offset on
the uncompressed page, because mlog_write_initial_log_record_fast()
does not do so.

page_zip_write_header_log(), page_zip_parse_write_header(): Encode the
offset in one byte.