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memtx: abort gaps after rollback

This commit fixes a Memtx-MVCC related bug that could lead to dirty gap
read in secondary indexes after rollback.

One REPLACE transaction (prepared but not yet committed) creates a
temporary situation where another concurrent transaction cannot see a
specific key in a secondary index (a "read gap"). When the first
transaction then rolls back, the previous (replaced) tuple becomes visible
again, and the "read gap" becomes irrelevant (the key becomes visible once
more). In this case, the second transaction, which read the gap, should be
aborted. However, it successfully commits, leading to a non-serializable
schedule.

This commit fixes the issue by adding the necessary handling during
rollback. Now, all such irrelevant gaps are aborted.

Closes #11802

NO_DOC=bugfix

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