rec_get_nth_field_offs_old(): Split from rec_get_nth_field_old().
rec_get_nth_field_old(): Reimplement as a constness-preserving macro.
dict_drop_index_tree(), dict_truncate_index_tree(): Note that rec is
an in/out parameter, as the B-tree root page number will be updated.
Replace ut_ad(mtr_memo_contains(mtr, buf_block_align(ptr), ...))
with ut_ad(mtr_memo_contains_page(mtr, ptr, ...)) in order to reduce the
number of buf_block_align() calls.
