memtx: fix definition of old story in MVCC insert statement preparation

During preparation of insert statements in MVCC, we define an old story and
abort all transactions that delete this story.

If there exists an older
story in the history chain, but the story is deleted by a prepared (not
necessarily committed) transaction, we consider that it de-facto does not
exist anymore — this logic is consistent, since during preparation of the
transaction deleting this story, the conflict resolution described above
was already done.

In this manner, there can be no more than one prepared statement deleting
a story at any point in time.

Closes #8104

NO_DOC=bugfix