btr_search_drop_page_hash_index(): Add const qualifiers to the local
variables page, rec, and index, to ensure that they are not modified
by this function.

page_get_infimum_offset(), page_get_supremum_offset(): New functions.

page_get_infimum_rec(), page_get_supremum_rec(): Replaced by
const-preserving macros that invoke the accessor functions.