do not allow extra_size to decrease on compressed pages.

Split page_mem_alloc() to page_mem_alloc_free() and page_mem_alloc_heap().

page_cur_insert_rec_low(): Remove parameter "tuple".  Implement some of the
logic from page_mem_alloc().

page_cur_tuple_insert(): Convert the tuple to a record and calculate offsets.

page_zip_validate(): Assert that the page is in compact format.