Optimize BPS tree for point lookups

Add a new API method to BPS tree to remove the code like this:

	struct tree_mem_key tree_key;
	tree_key.stmt = itr->key;
	tree_key.lsn = (*itr->p_read_view)->vlsn;
	bool exact;
	struct vy_mem_tree_iterator mem_itr =
		vy_mem_tree_lower_bound(&mem->tree, &tree_key, &exact);
	itr->index->stat.memory.iterator.lookup++;
	const struct tuple *stmt = NULL;
	if (!vy_mem_tree_iterator_is_invalid(&mem_itr)) {
		stmt = *vy_mem_tree_iterator_get_elem(&mem->tree, &mem_itr);
		if (vy_stmt_compare(stmt, itr->key, mem->cmp_def) != 0)
			stmt = NULL;
	}

Start at the shown vy_mem_tree_lower_bound call and trace the BPS tree implementation and its callers. Define the new point-lookup API around the key, LSN, and exact-match check shown, then verify that existing lookup behavior and statistics remain correct with focused tests.