Add some const qualifiers to accessor and validator functions.

btr_pcur_get_up_match(), btr_pcur_get_low_match(): Add const qualifiers.

btr_pcur_get_page(), btr_pcur_get_block(), btr_pcur_get_rec(),
rec_get_start(), rec_get_end(): Define as constness-preserving macros.
In debug builds, discard the const qualifier.

page_rec_validate(), page_simple_validate_old(), page_simple_validate_new(),
page_dir_slot_check(), page_validate(): Add const qualifiers.

rb:626 approved by Sunny Bains