memtx: fix reverse iterators gap tracking

In case of reverse iterators, due to index limitations, we need to clarify
the successor tuple early: this implies that the successor's story is not
always at the top of the history chain, whilst we need to add the gap item
to the story currently present in index — fix this by reusing the
iterators' check logic to set the current iterator's tuple (which is
considered the successor) to a tuple in index.

CLoses #7409

NO_DOC=bugfix