box: check iterator position against search criteria

If the 'after' key is less than the search key in case of ge/gt or
greater than the search key in case of le/lt, the iterator either
crashes (vinyl) or returns invalid result (memtx). This happens because
the engine implementation doesn't expect an invalid 'after' key.
Let's fix this by raising an error at the top level in case the 'after'
key doesn't meet the search criteria.

Closes #8403
Closes #8404

NO_DOC=bug fix
NO_CHANGELOG=unreleased