mvcc: fix reverse tree iterators gap tracking

Apparently, the current implementation of `tree_iterator_start_raw` is
buggy for reverse iterators: instead of tracking gaps for successors of
keys, it tracks gaps for tuples shifted by one to the left of the
successor: reorder the code of `tree_iterator_start_raw` to get
the successor tuple prior to shifting done for reverse iterators and
simplify the implementation to make it more straightforward and thus
comprehensible.

Closes #7073
Closes #7113

NO_DOC=bugfix

Co-authored-by: Alexander Lyapunov <alyapunov@tarantool.org>