btr_cur_t: Move page_block to page_cur_t::block.

page_cur_get_block(), page_cur_get_page_zip(): New functions.

page_cur_position(): Add parameter block.

Remove many page_zip parameters, now that there is page_cur_get_page_zip().
Replace some page, page_zip parameters with block.

Add some const qualifiers to function parameters and remove casts.

PAGE_HEAP_NO_INFIMUM, PAGE_HEAP_NO_SUPREMUM, PAGE_HEAP_NO_USER_LOW:
New constants.

Replace some cursor code in low-level diagnostic functions with
direct management of rec, because buf_block_t::buf_fix_count may be 0
when the functions are called, and debug assertions would fail.