memtx: rework transaction rollback

When we rollback a transaction statement, we relink its read trackers
to a newer story in the history chain, if present (6c990a7b), but we do not
handle the case when there is no newer story.

If there is an older story in the history chain, we can relink the
rollbacked story's reader to it, but if the rollbacked story is the
only one left, we need to retain it, because it stores the reader list
needed for conflict resolution — such stories are distinguished by the
rollbacked flag, and there can be no more than one such story located
strictly at the end of a given history chain (which means a story can be
fully unlinked from some indexes and present at the end of others).

There are several nuances we need to account for:

Firstly, such rollbacked stories must be impossible to read from an index:
this is ensured by `memtx_tx_story_is_visible`.

Secondly, rollbacked transactions need to be treated as prepared with
stories that have `add_psn == del_psn`, so that they are correctly deleted
during garbage collection.

After this logical change we have the following partially ordered set over
tuple stories:
———————————————————————————————————————————————————————> serialization time
|- - - - - - - -|— — — — — -|— — — — — |— — — — — — -|— — — — — — — -
| No more than  | Committed | Prepared | In-progress | One dirty
| one rollbacked|           |          |             | story in index
| story         |           |          |             |
|- - - - - - - -|— — — — — -| — — — — —|— — — — — — -|— — — — — — — —

Closes #7343

NO_DOC=bugfix