memtx: refactor rollbacked stories

If addition of a tuple is rolled back while the corresponding
story is needed for something else (for example it stores a read
set of another transaction) - the story cannot be deleted.
Now there's a special flag `rollbacked` that is set to true
for such stories, and the flag must be considered in places
where history chains are scanned. That approach also requires
psn to be set for rolled-back transactions, which surprisingly
not as simple as it to say. All that makes the code complicated
and hard to maintain.

There's another approach for managing rolled back stories: simply
set their del_psn to a low enough value (lower than any existing
transaction's PSN) and (if necessary) push them to the end of
history chain. Such a story would be invisible to any transaction
due to already existing mechanisms, that's what is needed.

In order to provide "low enough" del_psn it will be natural to
assign real PSN starting from some predefined value, so any value
below that predefined value will be less that any existing PSN and
thus "low enough".

Implement this more simple approach.

Part of #8648
Part of #8654

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit ba394a58)