btr_cur_optimistic_latch_leaves(): Use transactional_shared_lock_guard.

btr_cur_latch_leaves(): Avoid acquiring some page latches, because
the changes are already blocked by index->lock.

btr_cur_search_to_nth_level_func(): Remove a redundant variable
retrying_for_search_prev=!!prev_tree_blocks, and avoid acquiring
some page latches.